I know it's the opposite problem, but I have had trouble finding suits for my DD15 also that aren't overly skimpy in the chest area. Unfortunatly, the styles don't seem to be ideal for teens either way. Some of the suits available for larger bust sizes are on the frumpy side for a tween or teen. They want cute!
I found some tankinis at zappos.com that fit her beautifully, and were really cute, but again, her bust was practically falling out of the skimpy chest area. (Who's designing these things?

) I almost sent them back, but instead, I bought some spandex tank tops that they sell at Walgreens in complementary shades to the bathing suits, and she wore those underneath the tops. They covered her up reasonably, and looked like they went with the suits. She actually got a lot of compliments on them last summer.
This year, I got a jump on it early since she'll be doing a lot of swimming and boating this year. (The others are pretty faded from the pool, but she'll still use them for back up.) We were lucky to find a great salesperson at Pacific Sunwear (or something like that; it's a bathing suit store). She helped us find some really cute suits that fit her well and covered her up nicely, without use of a tank top underneath. They were a little more expensive than what I paid before, but if you add in the cost of the tank tops, probably not that much more. Worth the price, in any case.
Hope this helps someone. Good luck in your shopping!