I get mine from a local glass shop. it's called pyroceram and is used in fireplace doors. A piece 2"x9" costs me about 15 bucks. I've seen it on ebay also. Here is a link to where the guy is selling a platen and 2 pieces of glass. I've bought stuff from him before and he's a nice guy. If you contact him, maybe he'll sell you just one piece. I've also seen it on USA Knife Maker's site, but they always seem to be out of stock on about everything.
What do you use to fasten the ceramic onto the platen? Do you get any shocks?