https://www.amazon.com/Wonderbra-Womens-Ultimate-Strapless-Underwire/dp/B07GTGJGQ3

here’s a link. very reasonably priced and if you have amazon prime you can usually do a try before you buy. i got recommended it in r/abrathatfits and have since convinced 3 of my busty friends to buy it. everyone loves it! i hope you find a solution that works and have an absolutely lovely prom. the dress is stunning. :)