Very few, always judged on a case-by-case basis and roleplayed out. If there's an alchemist in town he might have a few potions for sale, high level NPC magic-users and clerics likely have a few potions and/or scrolls, a high level fighter might have some low-end magic weapons or armor, etc. But the players can never assume that magic items are available at fixed prices in unlimited quantities the way I generally allow them to do with non-magical gear -- if a player has the money and wants to buy 200 arrows or 30 flasks of oil or a dozen longswords he can do it automatically (cross of the money, write the items on the equipment list) but this doesn't apply even to the least of magic items. No one in my games can ever automatically say "I've got 3000 g.p., I'm gonna buy 6 potions of healing" or "I've got 3000 g.p., I'm gonna buy 20 +1 arrows."