Check with Plastics International in Eden Prairie. When I was a machinist at 3M we bought almost all of our plastics there. Definitely cheaper than McMaster-Carr.
A few years ago I bought a 2’x4′ sheet of 1″ black for $25, from a buddy who used to work at a plastic company, it was tossed in the trash. That was a jackpot find! Wish he found more.