I get mine from a small local lumber mill down the street from me. They cut it to whatever size I want. I generally get 3" thick white oak. It comes "green" and has to sit for several months to dry before use.
He charges me about 80 cents a board foot, which is way cheaper than Home Depot would charge for their "1 inch", which isn't anywhere near 1 inch thick.
So, look for a local mill and make friends with the guy.