I've bought plastics from Delvie (Salt Lake City, UT), NorVa (Norfolk, VA), and Pop Displays (South Bend, IN) for my prototyping. All three sell direct and also sell surplus through their Ebay stores (where I made most of my purchases). I've gotten really good service and really quick shipping from NorVa. Volume dealers like these don't like to retail small orders, but they usually have tons of smaller offcuts, to unload and found Ebay a good way to do it. If NorVa doesn't have what you need currently listed on Ebay, the folks that run their Ebay store will cut it and add it to your order- really good folks to deal with.