Well, I searched for sign stores near my house on google maps, and just about the only one that came up was a Sign-a-Rama. But when I was out today, I looked through all of the signs on the street and didn't see it. It may just be down a side road or something, though, I'll have to check this weekend if I can't find something else.
I did call two other home improvement stores nearby and they both just said they only have plexiglass, which would cost quite a bit for a piece that wouldn't even be big enough for the cage.
But besides all of that, I was just wondering if there were other materials acceptable to use.