This may sound so messed up but go to a shop or store that is dedicated to bras, panties and lingerie. It is pretty much like a wig shop in all they want is your business and repeat business. But if you are just starting out do not buy from wal mart or any other big box store. Get fitted if you don't know how to do that yourself and if need be then get one custom made. I suggest three at least if you have to wear them all the time. With three you can pretty much guarantee they won't start stinking from too much wear before washing them. And when you wash them hand wash them only.
Look we are trans women. We need to wear a bra a lot of times. So look up shops in your area that cater to trans women. There is no one that knows more about bras and finding your right size than another trans woman or someone that caters to them. You may have to travel a few miles but it will be well worth it. So going to a shop specifically is a good idea but remember them by repeat business every once in a while at least.
I mean seriously I buy shoes, jeans, blouses, Ts, tanks and even a couple of skirts and dresses at big box stores but my undies are closer to me than those other clothes and I want quality panties and bras. Even the lingerie I buy from small shops BTW and personally I would rather buy bras, panties and "naughties" from another trans woman or someone that caters to them. Usually within a four hour drive you can hit a trans specific shop somewhere. But I drove a truck most of my life so eight hours round trip ain't crap driving for me.

But seriously being trans compared to cis. With cis a 36A is the same as a 34B but we aren't cis women though. So I really would suggest what I just did about the trans specific shops or at least a custom shop. It does suck when you are top naked and someone measuring you no matter what but they have seen it before.
Maybe this will help with the different sizes...
http://cdn.shopify.com/s/files/1/0305/6249/files/size_chart-main_1024x1024.png?v=1500418658