More people should go crazy with their mirrors. Honestly, its the one thing you can hang on your wall, other than great art, that people will "wow" over when its unique and different.
There are so many great places to look for something different (and, if you shop around, you can get pieces for amazing deals, too). Great places to shop for "unique" things are antiques and vintage dealers, a little shop that's going out of business or advertising an inventory sale, local mom and pop consignments, estate sales (I've also heard garage sales can yield some great finds- but I would seriously consider the neighborhood before you devote a Saturday morning to a garage sale endeavor!), on-line auction houses and antiques dealers, and, chances are, if you know an interior designer or decorator- they might have a few things sitting around and they can surely point you in the direction of a few local boutiques. (As an semi-related "share"...designers change their stuff out a lot. We buy things even when we have no place for them. We are always seeing things we *looove*. I would say we are doomed from the start of any shopping trip. If you are BFF's or "in" with a designer, and you love something of theirs...its worth it to express a liking. That way, they know who to give it to when they are done with it or want to switch it out).
Tips for buying an already loved mirror: measure your wall space before you shop and remember it can be refinished! I don't recommend this for important and astronomically priced antiques (usually means they are important or rare or both), but do remember you can have it lacquered, leafed in a different precious metal or refinished with a metallic or solid color.

"Italian Brass Mirror" from Monument, $4,800
"Neoclassical Vintage Mirror" from Monument, $3,600
I love this one!

"Monumental Hollywood Regency Glass Framed Mirror" from Vermillion, $3,900
"Carved and Guilded Mirror" from Todd Alexander Romano LLC., $7,800
"Gilt Iron Mirror" from Galerie de France, $4,100