Just do it yourself.
I had the most difficult time finding a maternity swimsuit that was anywhere near decently modest. So, I found a style at walmart.com that looked like it might work. When it was delivered, though, the top was still too low. Unfortunately, I don't know of anyone who makes camis for swimsuits.
I googled "swimsuit modesty panel" and only found one tutorial using swimsuit fabric. It's $16/yd at Joann's. I figured I could make something work with an existing swimsuit. Took a trip to our local Wal-Mart store and picked up the cheapest black tankini top I could find, a ball-point needle for my sewing machine, and just went for it.
I pinned the tankini piece to the swimsuit top and trimmed away a good bit of the excess fabric.
Trimmed after sewing.