Our company manufactures UHMW Polyethylene in raw form -- boards up to 24" wide, 50 feet in length, 1/4"-2" thick. We can cut it to order.
One note for UHMW, however -- using it as a skidplate or undercarriage protection usually works well. UHMW has very good impact resistance, and is a very slick/low-friction material. Using UHMW where it will be exposed all the time to sunlight, however, can cause problems, as UHMW and UV radiation don't get along too well.
Depends what thickness you choose. IIRC, it is sold by weight. My guess is that a 4x8 sheet of 1/2" will be around $200.
