Hello,

Your cup size has nothing to do with the HRM you should buy, and really, so long as you aren't at the ends of the spectrum for ribcage circumference, the basic HRMs with the elastic strap will fit you fine. They go under the lower strap of your sports bra, tight against your ribcage.

I recently bought a Nike Imara HRM after a long stretch with Polar; I just like the look of the watch better. Their reviews are quite good and you can find good deals on ebay or by using Froogle.