If you search for "sintra" or "pvc sheet plastic" you should get results. I know tap plastics has it, and I think us-plastics just to name a few.

Also, sign shops can order it for you. They will likely tack on some cost on top. I get scraps from a local sign shop, which they charge me for if they are larger pieces. Sometimes their "scrap" is a 3ft x 2ft piece.
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Mark