Depends on the stones.
Cutlery and More: Good source for culinary Norton, all things Chef's Choice, some DMT, lots of good stuff.
ebay: You'd be a fool not to check.
Hall's Pro Edge: My choice for Arkansas stones. If you use them, it should be yours too. Not all Arkansas stones are created equal, it's very quarry depenedent. Hall's is tied for best soft Arkansas, but has by far the best surgical black -- as good or better than a Norton translucent.
Japanese Knife Sharpening: All things Hand American (one of the only sources), Shapton GS, Idahone. If he (Dave Martell) has it, buy it from him. Great guy, and one of the best sharpeners going.
Japan Woodworking: Bester, Shapton Pro, a variety of other man made and natural stones.
Sharpening Supplies: All Norton, All DMT, lots of other stuff; I use them a lot.
Tools for Working Wood: Only source for Naniwa, also has Ice Bear and Norton. A very good guy.
When you buy your stones don't forget to buy a flattener and a nagura.