Only take lumber outta dumpsters if it's in good shape. Don't use any sketchy shit, you dont want it breaking. But if you can find good lumber in a dumpster, take it.
It would be hard to find screws for free unless you stole them (not coo foo)
P.V.C. is extremely easy to find for free. The stuff is just asking to be taken from random places (but don't take it from like people's houses or whatever haha)
If you can't find free construction fence then, I'm sorry, but you're just plain dumb (jokes)----------------------------------------------------------------------------------------Also, get creative. Look around your yard and garage for things that you already own. (Example: You can make a drop in out of a playground, deck, treehouse, etc.) Doing this will save you tons of money.
Hope that helped d00d