I'll bite.
While I know that some get tired of the WCI Wood Supplier Mantra, THE source for basswood is:
http://www.heineckewood.com/
I just started carving less than a year ago so I can relate to your dilema.
I got a bunch of different sizes, for whittling, in-the-round and relief carving.
I got the largest pieces (more or less) I could imagine needing and cut some of those up with a table saw at work to make smaller blocks and plaques to give myslef even more ready-at-hand choices in my tiny workshop at home.
I also bought a few pieces of hardwood elsewhere, Itasca wood I think, to do other stuff with as time and experince progressed...some red oak, walnut and cherry, just for fun.
Lastly I bought a selection of beautiful butternut from Brian Bailey at:
http://www.baileyswoodworking.com/butternut.htm
I also cut some of that up at work.
Since I have yet to settle on one particular type of carving this selection is really handy and I find that raw material is not a limitation. No matter what my interest turns to I probably have a piece or two that will work, or can be cut to size to work.
...Well, you asked! ;-)
RussL.