The plastic can be easily found in hardware stores in the signage areas
(forsale, open/close, etc...), but be aware that those signs tend to be very thin, so when you form a peice you will probably want to re-inforce it afterward with that stuff.
Additionally you can get foamed pvc in sheets, its known by a few trade names like sintra and foamex. The various 3-4 mm thickness sheets can be good for vac-forming.