Even if you manage to find something on Ebay, try and price it in the department stores so you know how much to pay. We bought DD's dress for less than $30 (RRP about $60) in Myer and the exact same dress sold for $50 on Ebay. Granted we bought it during a sale but still!

The other alternative if you can't find what you want is to get someone to make it for you (if you know anyone who can sew!).