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That probably means you are not a 36C. It may be the size you should wear according to the tape measure but if it doesn't fit right it is still the wrong size. A lot of girls and women wear the wrong size because they choose their bra size according to a tape measure and not according to what actually fits right. You are probably wearing too big a back size and too small a cup size, for every back size you go down you normally have to go up a cup size to compensate. Your bra band is supposed to fit snugly under your bust without riding up at the back, or conversely cutting in. The cups should enclose your bust and the underwire at the centre of an underwired bra should sit flat against you breast bone, not poke forwards. There is some fitting advice on this website that should help you understand how a well fitting bra should look on: http://www.bravissimo.com/perfectfit/

I don't know where is a good place to find a good sports bra in the USA. Maybe a big department store? Anyway, wherever you go try on different sizes when you get there until you find one that actually fits properly and does the job. That bra will be your size, even if some simplistic algorithm based on tape-measurements says different.

You could try wearing two sports bras (sounds weird but it helps with supporting).

Try asking a female employee wherever you shop for clothes, she might be able to help.

If not, go on the Internet and ask around, that might help :D

New Balance store. A running store.