we buy all plastics locally from here
http://www.fortcollinsplastics.com/products.html
they sell smoked polycarbonate in .03 inches thick, we typically spring for the slightly thicker .06 so it doesn't tear out of whatever mounting holes are used. i don't know if Fort Collins plastics will mail order but it might be worth a shot. they will also lazer engrave it for you if you like, this costs more however.