one of my clients in wingdale NY has free plywood it is for pickup only they put out a pallet of it or so a week it is cut offs from homes they manufacture i picked up a few pieces of 1/2" and 3/4" that are 2x4' they also have 1 foot wide pieces from 3-8' long and a bunch of triangles various sizes.

if you have a modular home manufacturer near you they probally have the same kind of stuf the cut off 2x4's are burnable but they have a hard time getting rid of the plywood