There is a magical place that I often go to when I am in need of bits and pieces of junk or something for a set. Heck I even go there if there is some remodeling that needs to be done around the house. It's insanely cheap and there is often something for everyone who is either a prop builder or set designer. The place is called ReStore which is a store that Habitat for Humanity International owns and runs. The store gets all of the left over building materials from the houses that they build as well as donated materials from other builders and corporations.
In any of the ReStores you are likely to find plywood, barely used house paint, doors that cost two bucks, and rows of little odd bits and pieces. There is so much there that I cannot possibly list them all, you have to go and see it for yourself.
The best part of ReStore is that all of the funds goes straight back towards Habitat for Humanity so they can build a home for less fortunate.
I myself often go there before a trip to the hardware store, because I know I will find what I need at a cheaper price, and I feel good knowing it's going towards a noble cause. This past weekend I spent less than ten bucks and came out of the place with a shopping cart full of stuff.
To find out if there is a ReStore in your area check out their website here. Also for any of my Canadian readers out there you will be glad to know that there are branches on your side of the world.