I have been using a basic strapless bra that I have found at Walmart for about $6. (Can't remember the brand name) The shape and structure seem to work well and I add heavy peltex interfacing to the existing bra band to make it sturdier and non-stretchy. The cups can be made a bit stiffer with a couple of layers of Stiffen Quick found at your craft/fabric store. It seems that strapless bras have a better structure.
I also used a Victorias Secret "Secret Embrace" bra once and that worked great, although kind of expensive. I had purchased it as a regular bra to wear and thought it was too structured for daily wear. I made a great tassel/tribal bra out of though
I tried the sugar petals cups that you can buy and decided I didn't like them after making a couple of different bras with them.