Robby
I assumed you meant 12" x 12". I found 11" x 14" and smaller sheets of .020 to .080 styrene at a hobby shop. They're manufactured by Evergreen Scale Models, Inc. Woodinville, WA 98072.
They may make larger and clear pieces.
At one time I bought 1/2 a large .030 sheet (4'x4' I believe) from a local chemical/plastics supplier. I'm still using it. (It's kind of like buying 1/2 a beef).
Slightly different topic: I used to struggle to make good molds (plugs?). I just used a new technique to form the oil tank for a Tiger Moth. I shaped hard balsa, sanded very fine (400 W/D), then coated with 3 coats of finishing epoxy. Then fine sanded the epoxy surface. I used vaseline as a release agent. The piece came out very smooth. I still have the mold so I could make another.