Yandy.com recently listed six pony costumes that, while not named after the Mane 6 (for obvious copyright reasons), are a clear tribute to them:
If these costumes seem a bit, um, revealing, there’s a reason for that: Yandy.com is an adult costume store. (And by adult, I mean “Adult.”)
Something tells me that a brony is going to force his girlfriend (assuming he has one) to wear one of these for role-playing. (And by role-playing, I don’t mean D&D.) Worse: he's probably going to be wearing it. (Have fun with that mental image!)
Personally, I’ve seen far better costumes made by cosplayers. What can I say? You can’t beat costumes that are fan-made than those that are just made for fans.