I work in the intimates department of a major department store. In fact, my department is the 2nd largest intimates department in the entire company. You have no idea how often I fit and select bras for brides. I NEVER recommend NuBras to anyone. They simply do nothing for shape or support, regardless of cup size. Even though I warn against them, many still get them. I have never been told by any of the ladies who purchase them and come back later that they come completely unstuck, so if you want one the only advice I can offer is to also use body/dress tape to hold your dress in place. Sometimes this helps keep the bra in place. Again, I think that shape is very important in formal attire because it can completely change the look of what you are wearing. A lovely dress can be ruined by wearing the wrong undergarment.
I have two recommendations for longline strapless bras. The first is made by Lilyette, and it does not have any exposed seaming. The boning is completely covered; it will not show through. It is $36 and only comes in black and white. I believe it is available in sizes 34B-40D (no 40 in B cup though). My favorite longline is made my Wacoal. While it does not have covered boning, it is the most supportive longline I have ever seen in action. It also has a lower cut back to allow for dresses that have lower backs (but not completey backless). I call it my "Jump Bra" because you can jump and dance in it, and the girls will stay put. Also, the cups will not collapse on you (if you are busty). It is $74 (at my store at least), comes in white, ivory, and black, and sizes 32-40 B & C and 32-38 D & DD.
If you must have a completely backless bra, try the silicon paper stick-on types if you are between a large B – an average D. They take some practice applying, but they work ok. If your girls are smaller or larger then that, most of the stick-ons, even the ones made for petite and fuller sizes, will not work well at all.
Another option for backless (or nearly) AND if your dress isn’t strapless is a bra with a clear band in the back. Many brands make clear straps, but the only brand that I’ve seen that make one with clear straps AND a clear back band is Calvin Klein. It comes in black and nude, is convertible, costs $39, and is available in sizes 32 A-D.
Also, please do not rule out having cups put into your dress. Your seamstress should know if it will alter the fabric/cut of the dress if they are added. It is worth it to ask. I’ve never had a bride disappointed with how her dress looks on her after one of our store seamstresses has added cups.
Hope you find the above info useful.