Find a construction site and casually stroll on there and ask one of the workers for cut-offs. I do not recommend sneaking in during the dead of night. These guys seem rough and tumble, but most would love to help you out with a free piece of scrap. Their scrap is usually wonderful stuff in reasonable lengths, and the price is right. There will be certain times during the process that it will be available. They clean up often. The best times are just before and just after the concrete is poured, and just before the outside walls go on. The sizes you want are most likely found on commercial sites rather than residential sites. Electrical PVC is best- it is gray. The white stuff is probably ok, it is used for plumbing. you'll need to notice the wall thickness. Schedule 20 plumbing pipe will probably be a disaster for your project. I am thinking you will need schedule 40 or better. Usually the schedule info is printed along the outside wall. If it has been handled roughly, the info may be missing. Acetone will take the ink off when you want to make it pretty.
Also, if you drive by an electrical supply company they have damaged lengths. The stuff is often run over by a fork truck. The other end is still perfectly ok. They will often give the stuff to you jsut for hauling it off. there again, you only need a small piece. I have been successful in scoring gray PVC both of these ways.
