Does anyone know where I can get mirror cake plates/stands? I have a bride who wants a mirror under every layer of her cake, but it's a square cake. Or are there any ideas on what to use to create a similar mirror like effect between the layers?

http://www.ikea.com/us/en/catalog/products/15302600
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I posted these mirrors from Ikea. They are 4 in a set but just thought that maybe you would want them in different sizes right? have you tried buying a large mirror and taking it to get cut to exactly the size you want?

I don't know if they have improved over the years, but the last time I tried mirrors from IKEA, they were very thin and very fragile. In fact, I had to return the ones I got 3 times because they would break before I could even get home. I finally had to forget using them. You may need a heavier glass or you could make your own using plexiglass and the peel and stick mirror stuff from Home Depot. It's tricky to work with though. (It's the stuff they promote to use on your home windows to reflect sun away, but allows you to still look out without interference - we actually made a bottomless pit for Halloween using this method.)

I have used scrapbook paper that has a mirrored effect on it. I used it on my steampunk cake in my pictures on the bottom tier. I don't know if it would work in the situation, but it could be an option if you can't find square mirrors.

You can buy mirror tiles at stores like lowes and home depot. I wonder if they would work for you. They can be cut.

I buy them at lowes.

they also sell them at hobby lobby

Google for your nearest florist supply shop, or perhaps a party/event supply place. They usually have lots of options!
HTH,
Scott