usplastic.com has it. go to the sheets and whatever section, it's under the styrene catagorey. They have the measurements for thickness inches. I think .90 is about 1/8" thick. Also look in the phone book for any plastic dealers near you. You may be able to get it cheaper form somewhere locally. Here in Kansas City I go to Fantastic Plastics and can get supplies cheaper there. I know that doesn't help you, but it might help someone.
I find usplastics to be reliable too, never seen em outta stock, and my orders arrive quickly. gotta kill the fed ex guy though.. i saw him drop it in the driveway and chipped my dang sintra...its not reaaal bad though.. so i didnt send it back.