Undergarments

Check with your seamstress to see if she's able to put bra cups into your dress. My seamstress is doing that with my dress since my back of my dress is v-shaped

I had an illusion back on my dress, and my seamstress sewed in cups. I am not very gifted in the chest area, so it was just a little bit of a lift, and some padding to keep the girls in place.

If not look at the sticky bras if you are ok with those. I have a really good one from LaSenza which sticks so good even while sweating.

I got cups sewn in mine so i dont have to wear a bra at all

I agree with Tori, I would ask your seamstress if there's anything they can add to the dress or if they have recommendations.
You could also ask the store where you purchased your dress, they might have a few favorite brands that fit your needs.

Would your seamstress by chance be able to work his/her magic? I had an illusion back and (very) large boobs so a bra was not an option and she was able to make it fit in a way that my boobs were fine