Hayley, I'm sure you could get a great dress off ebay but make sure that you check their feedback carefully. If you put their username in Toolhaus.org it will give you all their neg and withdrawn feedback so if they have a history of ripping people off or selling dodgy merchandise you'll be able to spot it easily.
And if you pay a fair price for postage and pay via paypal with your CC you're better protected if there are dramas.