Another option might be mirrored plexi but again you really need a good substrate to keep it from getting all fun-house on you.
I used this once during a production of A Chorus
Line and Van is right -- that stuff is hard to keep
flat. That said, it's available in 4x8 sheets, so it's a good "wall" option so long as you can keep it
flat. We never did come up with an adhesive safe enough for attaching the stuff. Mirror Mastic is great for glass mirrors, but damages the acrylic to the
point where you can see the glue lines in the
reflection. We ended up sandwiching it between 2x2 frames, but alas, we had to live with the "fun
house" look.
In hindsight, maybe a silicone adhesive would have worked, or maybe even a
roll on contact cement. Something you can apply evenly to every square inch would be best.