Another good source of free building materials is "roadside loot", especially if you drive a pickup. I've found whole sheets of plywood, sheathing and a really nifty coarse weave polyester fabric among other things. Just keep your eyes open. Just a few weeks ago I found a perfectly good hand truck.
"One man's trash is another man's treasure." I just made that up.
I also "while driving" tend to notice "stuff" at people's homes. I will usually stop and ask them do they want to "get rid" of whatever it is I see. Most of the time, they tell me yes and I haul off with some more free stuff.