Marc;
If you're forming doesn't require a clear finish. A cheap source of styrene are "GARAGE SALE" or "BEWARE OF DOG" signs from the hardware store.
If you require clear I have found Squadron Thermoform sheets worked well.
Additionally if you can find a copy of the Kalmbach How-To Book Building and Finishing Vacuform Aircraft it has a complete coverage of the Mattel Vacuform and all its uses. Included in the chapter is the dimensions for cutting your own pieces with the actual Mattel dimensions. I found mine in a second hand shop for .30 cents and it is well worth that. I hope this helps.
Cheers;
Gregory