Quote from Mr_Rogers:
From much practical experience I would highly recommend "plastic cardboard" as a target/target backer material. This is the stuff that is used as advertising sign backing material, real-estate signs etc. It is about 1/8 inch thick and, if you look at the edge of it, it looks like corrugated cardboard. It is pretty easily available in 8 foot by 4 foot sheets.
This material is waterproof, lightweight, easily cut, and it will take hundreds of rounds when used as a backer for paper targets. We are still using target backers with an 8 inch diameter hole shot completely out of the center of the target but there is still plenty of area to staple new paper targets in place.
If you want a bunch of this stuff just wait until after November and pull all the political signs out of the trash that people put there after the elections. This isn't meant to be a political post but I am just telling you where you can get this stuff for free. I still have a few left from the last election and will re-stock this fall for the upcoming years.
The corrugated plastic is a great target backstop for everything from airguns up to the largest centerfire.
Just trying to help people out.