Not sure what your budget is, but you can try Macy's or Lane Bryant and I second eBay. You can also try Googling "plus size bathing suits" and see what you find. The brand I have is "White Stag" and it's well-made, but I don't remember where I got it.
GOAL #1: 10% (319.6 lbs.) - ACHIEVED 06/13/14 / GOAL #2: Under 300 (299.9 lbs.) / GOAL #3: 20% (284.5 lbs.)