Our ARES group had a grabNgo bag show and tell the other night, and one person had a "signal mirror" that was one of those 'business card' CDs. See
http://www.blank-cd-cdr.com/p-bcjc100.html
for samples, if you haven't seen them.

He gets them free from people, and they have a hole in the center and the reflective surface on the data side.

I also have a few 3-inch mini-CDs that some software came on. Same hole, same reflective surface.

They aren't as sturdy as polished metal, but they're free and very shiny. Regular CDs would work, too, but are larger.

Phil