I'm looking for a modern and high-fashion dress for prom, where can I go about finding one?

I don't really like the idea of wearing a 'traditional' long and puffy type prom dress so I was thinking I wear a more 'modern' dress that is still nice and dressy but not a little different from traditional. Where can I go about finding one? I was thinking maybe knee-length and wear fab platform type shoes which follow the trend. Does anyone know any places in London where I can go? I'm looking to stay under �100 for the dress... I've tried Ebay but there's not that much out there... also, has anyone got any good websites that I can look at for inspiration.