Great cosplay and it's nice to see a beautiful woman who cospalys and actually enjoys it.
As for the backless bra, you might consider doing the adhesive or "invisible" bras which are the cups that have the straps in the front to hold them together to help with lift and cleavage but don't have any straps. You can usually find them at stores like Jo-Ann's fabric or Hancock fabric type places back in the section that has the sew in shoulder pads and accessories for making your own bras and such.
Here is a link to their foundations section which shows a lot of what they have... [link]
I always look for stuff like this to use in photoshoots and when creating my own outfits and such to have models wear for when I shoot my concept/creative ideas. Does great for a lot of other things like cosplaying as well.