I had a couple of really nice dresses from Pumpkin Patch that I bought really cheaply (like, $20 or so) from their outlet store and they lasted me right through (and I was ginormous). I found the sizing a little big - eg I would normally consider myself a Medium but their Small seemed to be right for me. Try Dressmart, or use the Pumpkin Patch codes database to find dresses like
this one - you can get a reasonably plain dress and bling it up with some nice jewellery (and then sell the dress on TradeMe when you're done with it "practically brand new, worn once, $50 new, yours for $30" muahahaha).