If your on the lookout for something for a special occasion and want to make sure you never face the horror of turning up in the same dress as someone else head over to the amazing Juno Says Hello.
Their amazing website is full of some of the most people vintage finds. You may need to say some pennies but these are serious one off, keep forever dresses.
If it is way out of your league price wise then they do offer a hire service which starts at around £50 depending on the dress.
Take a look at their website, they have a pretty cool blog to read too.