This is my first time posting in this portion of Craftster and as a big girl I am ashamed that I didn't find this most awesome forum sooner. That out of the way...I think there is a possible solution for your particular problem..provided you are a bit flexible.
This year's trend seems to be the plus size bikini -- i've been waiting for this day to come, hooray! I saw an awesome blogger off of skorchmag.com who created her own bikini by using separates in different sizes (and different companies) to achieve the look that she wanted.
This seems to be a great way to solve the problem of being one size on bottom and one size on top ( i think MANY women have this problem, so take heart).
If you want to make a custom suit, zenaswimwear.com is a great place to start. The one downfall about this company is the lack of updated prints-- to me a lot of them seemed circa 1992 but the solids are awesome and if you are super picky you can buy swimsuit material in a print of your choosing and send it to them so your suit is fully customized!
You can have a custom made swim top and buy the bottom from any plus size clothing company (lane bryant, ashley stewart, torrid, etc) Even ebay can be a great source of swim bottoms that won't break a modest budget.
If you are a bathing betty on a budget like many of us are I would consider combining options from different companies.
For this you will have to be on the lookout early in the season or even BEFORE the season begins because I personally think swim tops with a smaller bust will be in high demand but short supply.
Lane bryant has come out with a collection of tankini and bikini tops that have underwire in them. At the moment the colorblock plunge bikini top is available in a B cup
You could combine that with any number of swim bottoms.
The most important part is not to give up. Wishing you best of luck!