As several people have said, in my campaign the one-shot items are fairly easy to find from alchemy shops or from a local spellcaster. Even one or two herbal shops have had the occasional potion or whatever.
In one kingdom there is a series of alchemy shops run by the school of wizards. They don't "have" magic items for sale, but they know just about anyone who has an item, and will act as broker, often for people who wish to remain anonymous as sellers (after all, you don't want everyone to know you just sold grandfather's heirloom sword, do you?).
When PCs wish to buy, they approach the shop, the owner checks his contacts, and frequently an item "similar" to the one the PC wants can be found. Frex, for a person who wants a bag of holding, they may be offered a handy haversack, or a pouch of holding, or a glove of storing. Since my campaigns rarely go beyond about 10th -12th level, I've never had to deal with giving out highly powerful items. But they certainly would not be available for purchase, or trade, nor even likely by commission.
Gilladian