If you're talking about the "plastic cardboard" material. It's called corregated plastic sheet and it's available here. Corrugated Plastics . This link is for 48"x96" sheets 6mm (1/4") thick.
You may be better off checking with your local sign company due to shipping cost. I'd also add a layer of 1/2" or 3/4" styrofoam sheeting on top of it when installing it. You're probably talking $500.00 or so for parts.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!