FREE stuff:
Go to a sign making store. Every town has one. They use tons of plastic sheet. They have a lot of scrap that you can usually get for FREE.
Go to a picture framing store/poster print shop. They have lots of scrap foam core. They will usually give you the scrap for FREE.
Go to a plumber/heating and cooling company. They have lots of scrap PVC. You can usually get some for FREE.
Go to a shoe store. You know those little plastic sticks they place in women's dress shoes? Those are used to hold the form of the shoe while it is in the box. Those make great pistons and/or other supports. They will give them to you for FREE.
Go to a computer repair store. They have lots of broken PC parts; fans, circuit boards, shells, wires, ribbons, etc. They will give you broken parts FREE.
Go to a 1 hour photo store. Their rolls of paper for printing photos come on these really short, really thick (1/2") cardbaord tubes. The tubes are great for scenery, but they also have a plastic circular disc on each end. Those discs are great for scenery and giant gears. they will give you the cardboard roll for FREE. Also ask them for any disboseable cameras they have gotten the film out of. They throw the camera away. It's lens is a great port-hole window for tanks or ships. It also has other gear type goodies inside.