Heyloo,
I'm looking for dresses for my high school leaving prom atm too. Try ebay you can sometimes find really lovely vintage dresses but also there are some ebay stores where you can bid for them to make you a dress and send it to you.
Also try department stores like John Lewis or Debenhams, i was rummaging around the sale in debbenhams and found a gorgeous dress that turned out to only be �18 i've not decided if i'm wearing it yet but it was such a bargain i had to have it! Try in there cause there are some amazing dresses, they've got a website too so u can have a look online!
hope this helps
Rach x