First of all let´s get this straight: if you have anti-gravity breasts that can support bras (instead of the other way around) you can buy any bikini top that catches your fancy. Most of us, however, need bikinis that support our breasts and give us a good shape.
Here are some tips on how to find them:
Buy bikinis in your bra size
Believe me, buying a perfectly fitted bikini top makes a huuuge difference compared to a triangles-and-string or a generic 75C. Right fit means breasts lifted away from the waist don´t look like they are a part of your belly, so you look optically slimmer. Good support means you can run, and body-surf without breast-bounce and without having to adjust the bikini. Right fit and support doesn´t mean a miracle suit or other expensive nonsense, it simply means the right size. Forget H&M bikinis in sizes like 34B and 36D; brands like Freya, Panache and Fantasie do bands starting at 28 and 30, and cups like J. Interestingly, most Freya bikinis have wider underwires than the bras.
The band doesn´t need to be super snug
Measure yourself correctly, but remember that in bikinis usually you don´t need such a tight band as in bras. With my 69/79cm measurements I normally wear bra size 28DD, but with bikini tops I usually go up a band size, and end up with 30D (going down a cup size because cup is relative to band size). I wouldn´t go any higher with the band, since I´ve already documented the time I´ve had to tie knots on a 34 band. Update: if you actually swim a lot. run and play sports on the beach then you would do better with a snugger band! If you prefer to read a book and get brown, it is not so important.
Brands / Stores I recommend:
Figleaves (US & Europe), Barenecessities (US), Bravissimo (Europe), Brastop (Europe, discounted), Ewa Michalak (worldwide) and Ebay UK are some of the stores I like. But you can also simply do a google shopping search for bikinis in your bra size.
Saving on bottoms
If you find 25€ a lot for a bikini top (and don´t like any of the dirt-cheap offers on Ebay), you can save money on bottoms. You can buy matching or contrasting bottoms in H&M and the like. The pieces can be pulled together by sewing on a matching button, cord or bow in a strategic place.
My favs of the moment:
(Most of these are available in a wide size range, and some in several colours / styles)
Top row: Pour Moi Azure Padded bandeau, Panache Taylor, La Senza Sling Halter
Bottom Row: Freya Vodkatini, Freya Jellybean Bandeau, Freya Carousel