Sustainable sheetgoods?

I need to find a piece of wheatboard or other “green” sheetgood (either 1/2” or 3/4”) to make a one-off stool. There’s lots of info on products like these, but mostly for manufacturers who are buying wholesale. Where can I go to get one 4’x8’ sheet? I don’t think green boards like this have hit the retail market yet. Any other suggestions would also be welcome!