Depends on if your looking for an original GI or a really nice reproduction. I bought a stock for mine from Numrich and its gorgeous good grain good color and accurate shape. I believe it was made by Boyds who also offer a reproduction stock in Walnut.
Not USGI, but nice, you might check out Boyds. They, according to their site, make a nice aftermarket one for the M1. No experience on that particular model, but Boyds has always provided a good product for my various projects. Godspeed.