Here is a suggestion to the original poster, while I do not have your issue, quite the opposite, my oldest daughter is on the large size up there. She went to a local bridal dress store and was measured properly for a bra, now she is wearing the proper size bra for her size, I was told how to measure myself and found I was wearing to big a cup and went down from a C to a B and my how much more comfortable for me. Point being is if you can not find someone to measure you in one of the stores mentioned, you might try making an appointment in a bridal store or an upscale ladies dress shop, they certainly will measure you and sell you that first well fitting bra.
Good luck