Any city will generally have one magical supply shop. These shops are where you can buy enchanting quality items, mostly for scrolls and spell books, alchemical supply items, and so on.
A very large amount of papyrus, parchment, and vellum will always be available. Most shop keepers will not sell their entire inventory to one person, or even a large part of it, so you can assume you can buy at most about a dozen pages at a time.
A very large amount of spell ink is usually available. These cost 100 gp per level of the spell being written; there are different inks for each level of spells. Again, generally no more than a dozen or so bottles would be sold to one person at a time.
A very large number of goose quills are generally available. A rather finite number of harpy feathers (the standard type of feather used when making books or scrolls) can always be found. On rare occasions (one check per month) a shop might have some of the below.
feather, pegasus 10% - 100g each
feather, cockatrice 10% - 100g each
feather, cockatrice 8% - 200g each
feather, hippogriff 8% - 200g each
feather, owl bear 8% - 200g each
feather, couatl 6% - 300g each
feather, chimera 6% - 300g each
feather, griffon 6% - 300g each
feather, sphinx 4% - 400g each
feather, ki-rin 4% - 400g each
feather, shedu 4% - 400g each
feather, roc 2% - 500g each
feather, phoenix 1% - 1000g each
Spell books are generally made to order, and take 5 days plus 1 day per level to prepare.
Some amounts of potion bottles will be available, and a seller would sell as many as 20 to one person. One level of a complete alchemical lab could be bought at a shop.
Such shops would also carry a large quantity of the cheap mundane items needed as spell components, but only things with a shelf life (no berries or living creatures). They would usually not have anything that costs more than 100 gold; such items would have to be found in specialty shops. All such items would be marked up quite a bit; you generally would spend a gold piece, minimum, for anything you needed there. (Note that I generally handwave such cheap spell components as part of your monthly costs, so unless you get trapped in a place where you can't find something you've consumed, you can ignore having to shop for cheap things.)