Most swimsuits are made out of lycra, or more commonly known as spandex. Pretty much any basic fabric store (Joann, Hancock, etc.) only provide spandex in solid colors, so they don't have any special prints or anything.
I actually got some spandex a couple of weeks ago from this website: http://spandexworld.com/
You can browse around all of the different selections they have, and you can actually order samples for free. Under the image of the fabric, it tells you what purpose the fabric is best for (swimwear, dancewear, blouses). Hopefully you'll be able to find something good, because I understand how difficult it is trying to find a nice bathing suit.