Have you checked out "Womanzone"? In my day (2006+7), they used to actually make lingerie and swimwear to order. Don't know if they still do or not, as I believe they've been taken over since then. You could choose your design and then fabric to have a costume made up to any special requirements, such as single or double pockets, padded cups etc.
Other than that, guess searching on-line, for "specialist or mastectomy costumes" is the usual option, Amoena, Nicola Jane, and such.
I find Mantaray swimwear is good. They have pockets in the cups with a thin modesty protector that can be removed. I'm not sure if they have their own website, but Debenhams stocked them (or at least it did, not sure since their recent problems). They have all sorts from one piece to tankinis to bikinis, and lovely bright designs. You may be able to find them on Amazon, too, but perhaps not the range.