I don't know what part of the country you live in, but here in the Midwest it is not uncommon for people to sell wood. Check with local wood workers and find out where they go. In larger cities there are woodworking hobby stores that sell hardwood, but often it is expensive there. If you ask around you will likely find someone that cuts wood for their own use and also sells some. If you are lucky they will have some large chunks.
Another place to look is at a farm auction. Many of the old farmers had a stockpile of rough sawn hardwood they kept after taking down trees. I have bought some neat, old, really wide planks at some of these sales, and severa are big enough to use for stock making.
Be aware that pieces big enough for a full stocked rifle are likely to be expensive. Clear hardwood is not cheap.
