Check the yellow pages for plastics. Local supply houses will frequently
sell off cuts, at a decent discount.
Also, ask them who is buying what you are looking for in quantity, those
buyers may be willing to sell you off cuts, and scrap. Sign shops are a
great source of lexan and other poly carbonates, in a variety of colors.
And, there is always ebay. You can get some good deals on some fairly
exotic stuff, (like graphite and molybdenum disulfide doped acetal), there.
It can be a real bargain if you find a seller close to home, so shipping
isn't too high. I got a 3-ft by 4-ft piece of a 1/4-inch thick glass filled
phenolic sheet, with a birds eye maple face for $30. It came from a shop in
Nevada that did custom interiors for high dollar executive jets. With
ebay, it really pays to do some research first though, so you have some idea
of value, before you start placing bids.