Stopped by a building being demolished and went inside. Had a hundred panels of old, quarter-inch plywood (almost always painted pale green on one side). I offered the guy a dollar a panel for each one he salvaged and removing nails. I loaded up my storage building with these 4 by 8 foot boards.
I just LOVE this good old thin plywood. It's a joy to work with, stronger than flakeboard crap of today, so light and easy to move into place. They just made the stuff BETTER, 50 years ago, than the crap of today.