plasticjunkie
I save those plastic cards that come in the junk mail to supplement my sheet stock. As others said, sheet plastic to fabricate the walls and gizmology from stretched sprue, thin wire and left over electronic parts to busy up the well.
Phil, another cheap source of styrene sheet is suggested to us by Hans Von Hammer: Use "For Sale", "No Trespassing", and other signs, that you can buy at your local hardware store. They're printed on styrene sheet, too.