RIP to people who find minor injuries a behavior deterrent but Lily's different

"Right, right. See the library first, pick out more books later," she agrees, setting the non-textbook carefully back onto its shelf with an appreciative pat. "I'm good here if you're done?"

"I think I'm good here for now, yeah."

To the potions corner of the alley with them! 

McGonagall considers the nature of the specific children she has come here with briefly, and then steers them without further inquiry to Mulpepper's Apothecary, where everything is clearly labeled and organized into multiple meticulously defined nested categories. He doesn't sell a pre-packaged first-year potions kit but he can and will produce a tailored recommendations list on request, complete with references, that covers the same requirement and then some, for anyone who can cogently answer questions about what they might need extra practice at or what they're interested in personally.

(She doesn't have anything against Arsenius Jigger, in fact she likes him quite a lot - he's staggeringly brilliant and she's collaborated on several papers with him and his standardization of the early potions curriculum has markedly reduced the amount of complaining she has to listen to from Slughorn - but the man would sooner burn his own house down than have something recognizable by any other human as a filing system.)

"Good afternoon, Minerva!" says Mulpepper merrily from his desk. "And new bright young minds, I see! What can I do for you?"

"What is THAT oh my gosh it's beautiful oh lookit the little labels - "

Lily is looking at the ceiling-high, iridescent-petaled two-hundred-year-old vine bush climbing the back wall of the shop, each of its individual branches sprouting some different strange nut or fruit or leaf or berry, neatly marked with a little parchment slip saying what it is and what it is for. She will be bouncing on her toes reading each one of them in delight for at least the next ten minutes, and possibly envisioning which ones she wants to acquire and cultivate in a windowsill.

"Oh, my.  This place..."

Oh, she's been asked a question.

"...Teach other wizards the value of a legible system of organization, perhaps?  I am in awe of your work, here.  Especially in contrast to everything I've seen in the other shops, it's a refreshing balm of peace and legibility in a society that seems to be optimizing for chaos.

"That said, we're mostly here for first-year potions supplies.  I think I in particular am interested in getting supplies that could equally be used for Muggle chemistry; there are some things I don't know if wizardkind has bothered to invent and don't want to - leave to chance, to have available, because I'll definitely need them.  ...That, and I generally prefer to work precisely, when I can."

"Why, thank you, my dear!" beams Mulpepper. "Unfortunately many wizards, including certain other apothecaries who shall remain nameless," he here winks in the general direction of next-door, with an air of fond exasperation, "are, indeed, not simply undereducated but rather choosing on purpose to be like this. That second thing, though, I can certainly help you with. Now, first off, you'll of course need your cauldrons and scales and vials! For most students I'd recommend a basic pewter but if you want to mess about with nonmagical concoctions you really ought to have something low-reactivity, chemistry in a pewter cauldron is asking for a hole in your desk - copper, perhaps? Not the very best option but, begging your pardon, most first-years haven't the budget for gold."

"I was vaguely assuming I'd be procuring - probably glass, beakers of some sort, actually; you are, however, correct that I haven't much money.  ...How does potioneering avoid the issues of concocting something in pewter, anyway?"

"That's the principal function of the stirring steps in most recipes." He gestures at the shelf of variously sized corked glassware. "Most students only require the smallest sizes, for submitting samples of their classwork, but for personal use perhaps a medium-small?"

She investigates.  "That seems like it might be generally sufficient, though I'll admit that I don't off the top of my head recall the specific yields of things; I just know the syntheses in question exist."

"If you find later that you've picked the wrong size I'm always happy to do free exchanges on glassware," he offers, since she doesn't seem inclined to explain what it is that she wants to make and it is certainly therefore none of his business to try to guess what its yield might be. 

(Glassware is very easy to magically sterilize and quite difficult to curse, so unlike many things it is perfectly safe to resell as-new.)

"...Hmm.  Perhaps I'll just buy the glassware for that project later, when I've had a chance to figure that.  Thank you, though, that's good to know.  ...Out of curiosity, is there a reason people don't brew potions in glass?  I'd imagine that magic is actually particularly capable of solving the bit where glass doesn't heat evenly; I don't know if there's any magic-side problems...?"

"Oh, potions don't like to be looked at below the surface while they're brewing," he says. This does not appear to be a joke. "Though focusing on the necessary thermal balancing charms while also paying attention to your brewing sounds as though it would be rather fraught, too, I must say."

"I was imagining solving this at the 'make a lot of smaller flames' burner level, admittedly.  Or maybe the same principles as an electric stovetop - actually, that's definitely an idea.  Anyway, as a matter of equipment, rather than in-the-moment charmwork.  Potions...Don't like to be looked at below the surface?  Huh.  Well, you're the expert, presumably.  Wonder why that is.  Maybe when I have time and money to spare on experimenting...But not now, regardless."

"I'll still be here whenever you're ready," he agrees, as various tools are accordingly purchased and packed away into trunks.

Lily picks out a standard student cauldron, cheerfully telling Mulpepper that she fully intends to destroy it after finals and will come back and get a nicer one next summer after she has carefully planned and safely executed the experimental creation of something educationally explosive.

(McGonagall sighs something under her breath that sounds very like an extremely fond gryffindors!,.)

Mulpepper, chuckling, sells her that among her required supplies, and then starts in on his spiel about consumable ingredients, which goes on a while and includes various commentary in the vein of 1) telling Ophelia the very minimum necessary quantities she could make all the standard first-year homework assignments with if she is very precise and doesn't waste anything, with a warning that he usually recommends most students bring at least twice as much, 2) pointing Lily to a few interesting ground peppers that wouldn't normally go in the first-year kit but that given her plans for her first cauldron she might enjoy, 3) advising them both that he gets a lot of owl orders for refills on belladonna, which is one of the most common ingredients in simple potions and remedies many of which students find they like to brew for personal use, and it's cheaper to stock up in advance since that avoids the owl order fee, etcetera, etcetera.

Ophelia certainly doesn't plan to waste anything, but one never knows what the future holds; she'll take the recommended stock, this year at least.

(...Perhaps she'll brew some of those remedies for resale, if she's confident enough in her work.  And if that's allowed by the rules; she wouldn't wish to find herself in trouble thereby.)

How very responsible of her.

If they don't need anything else, Mulpepper will send them on their way, with a very sincere "Looking forward to talking to you again next year!"

Ophelia gives him a small, but assuredly sincere, smile.  "I'm looking forward to seeing you then as well.  Thank you!"

With their potions and books collected (and a brief detour back to Madam Malkin's to pick up Lily's newly assembled uniforms) all that remains is a trip to Wiseacre's Wizarding Equipment for collapsible telescopes, which they're required to have for Astronomy class.

There will appear an additional detour when, on the way there, Lily discovers that there is an ice cream shop.

She rifles through her remaining budget - more than she'd have had without Ophelia's regular gentle reminders - asks McGonagall how much a telescope is usually - and declares with delight that they can totally afford to stop at Fortescue's for magic sundaes, and therefore, unless Ophelia has a good reason they shouldn't, they're totally doing it.

"...How about we stop there after we're done?  So we can take our time and enjoy it; celebrate a successful day."

"I don't want to take up too much of Professor McGonagall's time but if that's all right -?" (McGonagall nods.) "Okay, yeah, great plan."

To Wiseacre's!

The Wizarding Equipment shop is alive with inscrutable mechanisms, tiny clicking things, shiny polished surfaces, and so forth. They are reasonably well-labeled and categorized into groups by approximate use case - "Divination", "Home and Office", "Research", "Personal", and up front, "Hogwarts", which is entirely a shelf of little folding telescopes. There is a prominent sign by the door reading YOU TOUCH IT, YOU BUY IT. 

"...How enforceable is that sign...?"

She's going to...inspect the merchandise.  While carefully not touching it.