I have been professionally fitted by a custom bra shop. I get all custom bras because I'm small, like alot of us on here. I wear a 28E or 28F. I am similar to your size. I measure 27 under, and 33 around the fullest part. For every inch bigger you are from band size to fullest part, is equal to one cup size. And, you're band size should be the same or closest to your under measurement. My bras are expensive, but they fit like a glove. I have no issues with band being too loose or riding up and the straps are where they are supposed to be. There are some places online that sell bras in that size. But, honestly, I'd try to find a shop that does it properly.