RE: Vacuum Form Materials
The cheapest place to get sheet plastic is usually a local plastics supplier that has no minimum order, or one that will waive the minimum if you pick the plastic up in person. Call around. Some plastics suppliers have $100 minimums to keep riffraff like us away.
Others have no minimums, or a $25 minimum, and are happy to sell you a sheet or two. (Unfortunately, you generally have to call and ask about minimums; it's not usually in their yellow pages listing or on their website.)
A 4 x 8 sheet of thin plastic is generally cheap, say $15-25, because plastics suppliers price things mostly by the actual amount of plastic, and for a thin sheet, it's just not very much plastic. You can roll a thin sheet up and take it home, and cut it into the size(s) you want by scoring and snapping.
Anywhere else, you're likely to be paying much more per sheet (of whatever size) than you're actually paying for plastic.