Try going to TV repair shops. Ask them for old mirrors from projection TVs. They usually have precise front surface mirrors that are fairly thick,and can be cut down to any size you like. Free and inexpensive. =)
Last weekend I hit up the local TV repair shop again and the guy was more than happy to let me get the mirrors, as long as I disassembled the rest and tossed the remains in the dumpster. I got a few other good goodies from gutting them as well.
It was a fun and constructive, destructive morning =).
Will there be three phase!!!!