I think if you have a really good strapless bra (long line is best) that covers your bust completely, you will be fine. Then even though the dress is low cut, your seamstress can make it tight across the chest. They can even raise the halter part so that there isn't too much cleavage. I've had that done for a dress before. But it's all about the support, so with a good long line strapless bra, you should be fine. I just got one from Georgette's in Hicksville, and I'm a 34 DDD. It's a very good bra, from Goddess I think. But if you go there they will fit you with a bra with a lot of support.
try victoria secret & macys..they both have a good selection.
Do you mean your wedding dress? If not, then I have the same problem. I always find adorable halter dresses and can never wear them. I hope you find some good suggestions! As far as a bra for a wedding dress, definitely try the Goddess bra. that's what I have and its great. I can't wear it under a halter style dress though.
