IMC, we play in the Forgotten Realms. All the temples / shrines / whatevers of Mystra, no matter the size, are tasked with the same duties: (For a fee) Identify or Analyze all magic items requested (usually the latter and usually for 1% of appraised value), and act as the official unofficial trusted agent for magic item sales.
My group usually deals with the larger temples (they're based out of Waterdeep now), and all the temples have means of communicating with and transferring items between themselves. So the first place the PCs turn to for their magic needs is the Temple.
Either the item already exists or the temple knows of someone who has indicated willingess to do commission work. The lower in price the item, the more likely it is to already exist, and items that can be upgraded out of are more prevelant than the top of the chain items. (IE, lots of +2 items available, almost never a +6 available).
For the Tomes and Books and Librims, I decided that only one place in Faerun has access to the +4 and the +5s: Waterdeep. There's a house in Waterdeep with a cross-planar Connection to another demension where Xp is handled differently, and there's a community that can generate one book a week -- if the prices are met, and money is only part of the price. In this case, it also requires a truly horrendous Perform check, but you could also take years off their lives or whatever pleased you and seems fair.
As long as I play in FR, I'll be using the Temples of Mystra as my conduit for magical items, just as the temples of Waukeen serve as my conduit for simple banking.
