Hmmm. I'm working on trying to build a similar UV box for use with alternative process photography.
But I used to get extra think glass to make contact sheets from the hardware store at 26th and Lyndale. It's now closed, but I'd start at a good neighborhood hardware store. If no luck there call Harmon Glass ( http://www.harmoninc.com/
) or try a stained glass shop. The folks at the Glass Endeavors are pretty cool ( http://www.glassendeavors.com