Your best bet may to check with your college where they get their scientific supplies from. Whenever I needed anything like this when I was at 6th form, I would usually browse through their mags and order from there
Some specialist scientific shops should have it too. How much are you looking for, a lot?
Not too much, enough to do about 3/4 LCD's so a 200x200mm sheet would be good. I've talked to my college like you said, they said they'd order some for me from "Yorkshire Supplies" or something so problem solved for now