Guess I'm lucky to live in the aluminum boat capital of the world. I can get sheets of almost any thickness, diamond tread or flat, up to 4x12. Normally shipping would kill you but if I'm not in a hurry for it, I'll have them send it with the next load of boats...that way they don't charge anything to ship it.
Last time I bought any, it was .080" thick flat (no diamond tread), 4x8 and was about $68 for the entire sheet.