Bra/corset advice for my wedding dress

So, I am getting married and I found my dress!! 🎉🎉❤ I haven't gotten it altered yet, so it doesn't quite fit right up top, but fits perfectly everywhere else.

The point of posting this is that they told me to make sure I have the bra/corset I plan on wearing for my wedding day so that they can alter it correctly.

The issue I am having is that I have 40DDD breasts and am going to have to find some sort of strapless bra/comfortable corset to wear. I have never owned a strapless bra because I am afraid it won't support me properly and I am afraid of being uncomfortable in a corset. I have no idea where to start! 🤔

They did say they could sew cups into the dress, but I don't know how I feel about that exactly.

So, my question to you ladies is: what do you suggest I try and where do you suggest I go to buy it?? I'm hoping for comfortable and functional. Any help would be extremely appreciated! 😊