Most yard signs are coroplast/corrugated plastic. 4 millimeter is the norm for most coroplast signs (especially yard signs), although for a thicker higher durability they make 10 mil coroplast.
You should be able to get a 4ft x 8ft sheet from sign supply companies. A decent sized sign supply company should have quite a bit of stock on hand for that, acrylics, aluminum, and more of the common used sign bases. Proveer/sign supply usa has a location in Tampa.
You may be able to find a few more via yellow pages under Sign Supply, just gotta make sure you're dealing with a vendor that deals only with sign companies and isn't a sign maker
