Tink, if it's for one night only, and you won't wear it again - try a fancy dress shop. If it's a good quality shop, you'll get a Fairy Godmother for a rental fee.
Tinks, here in Edinburgh there is Butterflies on the Grassmarket, Emma Roy, Eleganza Sposa and Pretty Woman. Although they are more bridal, they also stock bridesmaid dresses, evening wear and prom wear. xx
If you really dont found your perfect dress yet i would like to suggest you one of my trustworthy shop where i do , Its a online http://www.gownsexpress.com .Its a new shop ,i shopped her around 1 month before for evening gown .