I dumpster dive for all kinds of stuff. I will add this to my list.
We get aircraft sheetmetal on pallets where i work. Theres heavy duty paper placed between each sheet. I roll it up and take it home and use for auto body masking paper. Also under the bottom sheet protecting it from the pallet theres usually 1/8" thick masonite or a little thicker like 3/16" i used the 1/8" to make the trunk divider panels and package trays for both cudas i have, and the 3/16" to make spare tire boards for both cudas as well. Free, and all i needed was my jigsaw, a sander, and the old shot pieces for templates.
The pallets come with heavy duty corrugated cardboard flat sheet on top to protect the top sheet during transport. I save this too, because i have used it to construct special sized boxed to ship parts in.