How much capability do you have to size stock? If you have a jointer and/or planer, that opens up a lot of options in terms of availability, quality, and cost. If you don't have these things, then you are probably best served by Woodcraft, which has a small to medium selection of more exotic milled wood in 1/4, 1/2, and 3/4 thickness. It is much more expensive than buying rough or skip-planed lumber at a place like Austins, which is excellent, by the way, but if you are only going to be using smaller amounts and/or need it to already be milled, then Woodcraft may be your best bet. Sears Trostel up here in Fort Collins has a decent selection as well, although I think Austins has more variety.

If you don't have the stuff to mill stock to specific dimensions, but do have a bigger project or need larger quantities, let me know. You can always buy a plank or two of rough stock and I can resaw it and run it through my jointer-planer for you.