If you contact suppliers of gunstocks most will sell you cut off pieces. Dunlap always brings a bunch for folks to sort over when he's at shows selling stocks. Some folks like Chambers even have ruined precarves they may sell to folks wanting to practice. These are often used at classes teaching carving, etc.
It is also possible to go to the woodpile, take a piece of firewood out, chop it, plane it, and carve it.