Lowes, Menards, and Home Depot all sell hardwoods in various dimensions. Home Depot will sell some (oak and poplar I think) by the foot at about $5/bdft. Our local lumber yards do the same for hardwoods, although some won't let us leave a length of <8 ft on their shelves (you can cut up to 5 ft off of a 13 foot long board).
I recently went to a local cabinet shop in town. They use mostly birch, oak, walnut, maple, cherry and poplar lumber and always are leaving short board ends. I bought a mixed pickup bed full (about 500 bdft) for $100. Mostly 3/4 or 4/4. Setting up a relationship with them was a good thing. They have commercial planers and sanders and will do small jobs for a fee if you let them fit it in between their cabinet jobs.
Harder to find Baltic Birch plywood locally, but there is a specialty woodworker's shop about an hour away that sells it. It's worth the drive to have some 1/8" and 1/4" in the shop. (Note that all BB Ply is sold in 5'x5' sheets, so be wary if someone tries to sell you a 2'x4', 4'x4' or 4'x8'. Probably not BB.)