As We Change carries a line of swimwear specifically designed for mature women, including figure-flattering styles. They also carry coverups.
Blair.com features a broad selection of modest swimwear which is geared towards more mature women.
Chadwicks carries a broad line of swimwear, mostly appealing to mature women, including a "slimming swim" line.
Lands End features a broad line of modest swimwear for mature women in regular, petite, plus, and long categories - including slenderizing and figure-flattering styles. (Most styles are modest, but they also feature less-modest styles). They also feature swim minis, swim shorts, and coverups.
L L Bean features modest swimwear, including swimsuits, swim dresses, swim shorts, and swim skirts.
Orvis features a number of modest swim suits.