Sewable bra cups

The seamstress should have them on hand or purchase them herself. It's not your responsibility to provide them. Try them on first. Not everyone finds them to be supportive especially if you are top heavy.

We ended up buying several sets trying to get exactly the right thing for daughter's dress. We bought them at Hobby Lobby and JoAnn's in the sewing department, and I'm pretty sure I ordered a set from Kohl's (look in the intimates dept.). I may have also bought a set at Target.... So many sets of bra cups! lol Good luck!

Sorry! To the second part of your question. My daughter is thin, but a DD. Her dress had a lot of built in structure, but the bodice didn't fit tight enough for her to feel "secure" in her dress. Her seamstress actually used two sets of bra cups -- a set of push-up pad/cups just below her natural bustline and then a set of small pads sewn just below the top edge of the sweetheart neckline. As a DD, the LAST thing daughter ever expected was to need padding/push-up cups in her dress, but, honestly, they were a miracle! With the two sets of pads sewn into the bodice, her "girls" were held snuggly exactly where they needed to be. She was in her dress for 12 hours and was an absolute maniac on the dance floor, so she really tested that dress! Good luck!

My seamstress had her own, but you can get them at sewing/craft stores like Joann and maybe Michael's. If Amazon is functioning normally in your area (we had covid-related delays during our peak) you should be able to order some today and have them delivered on Friday. The cups themselves aren't very supportive, but the seamstress should be able to make the dress more supportive and the cups help with shaping.

I found some on Etsy (I didn't buy) but they had DD and larger cup sizes, but most of the shops had great reviews.

I found some sewable cups for darker skin tones on dressmakingsupplies.com

I’m still going back and forth on cups versus a low back bustier. Curse you gorgeous illusion details!!! (Seriously, my strapless bra sits just the tiniest bit too high and can be seen through the back of my dress. 🤦🏼‍♀️)
I know the cups would be cheaper, and I asked my cousin who is built similarly to me if she felt the cups sewed into her dress were supportive enough. Mind you her dress was strapless and mine is not.

I’ve never heard of having to find/bring your own cups in for alterations. I’ll have to keep that on my radar as well.

My seamstress just had my girls bring a bra with the correct sized cups. Band size didn't matter. She used the cups from the bra for their dresses.