Welcome to the forum.
My best source was a Craigslist post of a guy trying to make some buck from offcut acrylic at a lighting manufacturer. His offcuts were 24 by 36 and even wider. I have enough to last me for a while.
People have success with local plastic suppliers if you live in larger metros. Tap plastic to be specific.
There wood sources are all searchable on the forum.
Habitat for Humanity.
I’m a scrounge and don’t do too many production runs.
What you are most in need of is some type of metric to assist you in evaluating a resource. Plywood that is dimensionally accurate and flat and thin enough. There have been lots of posts about these sources. Not all plywood will fit the bill having voids, knots and glue densities.