depending on where you live, vintage shops can be goldmines.
Alternatively, purchase a basic dress in the preferred shape and visit
Barnett and Lawson for trimmings and all sorts of things to customize it. You can pick up noce dresses with vintage inspiration from most of the high street shops at the moment - Top Shop seem to have some nice 40s' and 50's inspired pieces.
Fairy Goth Mother in Camden is quite good for dresses too.
If you want something super special then check out
Eternal Spirits
corsetry - Susi there makes complete outfits and can create garments from any era.
Think accessories to get a 'burlesque' look - quirky hat, beads, tassels, seamed stockings.