Try ebay. You can usually find gunstock blanks pretty cheap (I bought one for about $40 a while back)
Some of these may still be green (still has excess moisture in it), so be sure to read the descriptions. If it says something like "it has been air dried for the last year", it's most likely still green. The problem with green wood is that it will shrink, and possibly warp. If you do get one, let it dry for a while longer.
You should be able to find an already dried piece pretty easily though.
Also, keep in mind that making a stock is a lot of work. You can make one without too much trouble with a spokeshave, rasps, and files though. It will just take a lot of time. (there are probably more efficient ways, but this is how I'm making mine)
Have fun, and don't forget to post some pics when your done.