My roommate is a 34F and also has trouble buying bikinis/bathing suits. (I'm also odd-sized up top, but there is no way in **** I'm wearing a bikini anytime soon so this isn't an issue.)
Anywhos, there's a Canadian company that custom makes bathing suits if you send them your measurements:
http://zenaswimwear.com/
They also do one-pieces and ship to the US. It's not cheap (about $60 per top) but it's well worth it if you want something that will fit and look good.
Also, after years of my boobs bouncing up and down when running/playing sports, I finally found an amazing sports bra that pretty much plasters your boobs to your chest so they don't move at all (I'm 36DD but it also works for my 34F roommate just as well):
Champion Sports Bra
Again, not too cheap ($42) but it's on sale for $25 right now. Great deal for a great bra!