Here's a link for edible silver leaf:
http://www.goldleafcompany.com/goldleaf.html.
Haven't tried it myself. Just googled edible silver leaf and this was one of the sources that came up.
Did you see the Food Network Cake Challenge where one of the cake decorators made a mirror using isomalt? I think it was one of the Walt Disney cake challenges. Maybe someone else knows exactly which show it was.
The silver leaf does exist, however it can pe pretty pricey.
I think that idea just might work though.
i think the show was something like Disney princesses. A guy made the mirror on the wall for snow white. i don't know if it was like a real mirror though. but he had a mask and poured melted sugar all around it into a mold. then took the mask out and put something else in it to make it look like the mirror. i don't know but that may be what you saw ptanyer.
It was actually the Shrek challenge, he made the mirror for Lord Farquaad. He had problems with the mask at first, but finally got it where it wouldn't float in the sugar. He then took it out and put in white chocolate for it.
Did you figure out the candy mirror yet? I'd really love to know how you are doing it. I'd lie to make one with a scary face in it for my halloween scare-0-thon. this year I'm making an entire edible room. So any more advice on the mirror would be great!
My preface is that I have no idea what I'm talking about...how's that for instilling confidence? 
Instead of the "pricey" edible silver leaf, could you use a very thin piece of gumpaste, painted with vodka and silver luster dust? It wouldn't be as reflective, I would imagine, but it still could have the "mirror" illusion.
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