well man, as for having worked for a plastics retailer and fabricating shop, just about all forming plastics (therm-form, press form, cast sheet) come in various sizes between gauged thicknesses of as low as .010 up to .200 which in turn can be converted to fraction measurements like up to 3/4 inch.
Now your typical plastics seen for vacuum forming are:
Styrene
ABS
PETG
HDPE
Acrylic
and sintra
All these usually are sold indivdually in a 4' x 8' sheet. Styrene will normally run about $20 a sheet where the prices vary up and down for the others. Your best bet is to find a plastics wholesaler around your area as their orders are usually in the hundreds of sheets in plastics. Look in your yellow pages under "signs". Most sign shops vacciform in some sort, so they probably have those plastics around and you could also ask I they have any flawed scraps big enough for you applications.
Now for the plastics around the home ie: trashcans, storage tubs are usually cast in polypropelene which is liquid die cast, which makes them extremly difficult to near impossible to use in vaccforming.
