It is really not that difficult to make a one piece swimsuit or a bikini. If you want your make to look professional, you should add cups and swimwear lining. The majority of sewing patterns have additional passages about how to add cups.
This pre-formed cups, which we carry in sizes S, M and L, are white and made of 100% polyester. They are most suitable for one piece swimwear. Of course the material dries quickly.
It is not quite trivial to allocate the swimwear cups to the real bra sizes, but the distribution is roughly like this:
Small: B- to C-cup
Medium: C- to D-cup
Large: D- to E-cup