Aw coolness a Nocturnal cosplay ! I wouldn't have the courage to wear such a costume but I hope you'll make it look great and hopefully tasteful too.
Anyway, I was able to find, with a very quick mobile search, stuff like these http://www.asos.com/Fashion-Forms/Fa...rt=-1&clr=Nude
Dunno anything about that particular shop or those as solutions; how or if they work (it said they can be worn with or without bra), but maybe that's a starting point? I googled with keyword invisible bra and only looked through this one site.