I found this stuff at a local discount wholesaler in Maine. It is marketed as some sort of window insulating pane to place over the frame inside a window as if an upgrade over the shrink-wrap stuff. It comes with the peel-off film to protect the surface, and the plastic is very transparent. The great thing about it is that it is very pliable and bends easily. I point this out since in terms of display cases, the choices are usually glass (hard to work with & can’t curve if home-made) or thick Plexiglas from hardware stores which is fairly expensive and also does not bend.
The brand is Red Devil, and it is referred to as “Snap-N-Seal” Optix Acrylic sheet. The ones I have are 42" x 64" and only cost me $11.99 each. The label also says non-yellowing, UV-blocking, and 10x stronger than glass.