Victoria's secret carries backless bras.
http://www2.victoriassecret.com/commerce/onlineProductDisplay.vs?namespace=productDisplay&origin=onlineProductDisplay.jsp&event=display&prnbr=9G-212026&page=1&cgname=OSBRPADHZZZ&am
p;rfnbr=5230
Basically, they have adhesive on the sides which you attach you your skin. I had to wear one for a costume recently and it worked very well. I'm an A cup, so I don't know how it would be for larger sizes. I'm sure it would work fine.
Or you could go with this bra, which can be worn 100 ways. I'm sure there is a good option for backless costumes.
http://www2.victoriassecret.com/commerce/onlineProductDisplay.vs?namespace=productDisplay&origin=onlineProductDisplay.jsp&event=display&prnbr=9G-211598&page=1&cgname=OSBRPCNTZZZ&am
p;rfnbr=4859
If you are having trouble, I recommend going to a lingerie shop and getting some advice from the clerks on how to get the best support.