Tend to buy tankinis these days, I find they fit me better as I am 5-10" tall , costumes are never long enough in the body length
They don't seem to have much in stock at the moment though the new season stuff will be in shortly, but if you are wanting a one-piece have a look at the following website:
http://www.longtallsally.com/?search=swimsuit
They specialise in clothes for women who are 5ft 8 over and this includes swimsuits that are made longer in the body. My Mum has always had this problem - even though she isn't tall she is very long in the body. One time when we were down in London I took her to one of their shops - all 5ft of her - and whilst the assistant was initially non-plussed and tried to tactfully explain that the shop wasn't really for the likes of us (I'm only 5ft 2ins myself!), my Mum was able to find the most comfy and best-fitting suit she'd ever had. Even the assistant had to admit that it fitted her perfectly! Those extra few inches can make all the difference.
These days, they sell over the Net and so they never realise just how short some of their customers actually are