If you want to measure you can do it yourself. There are plenty of videos and articles on the internent that help If you want to be buy a measuring tape I could guide you. Measure right under the bust, where the band goes and write down that number. If the number is even add 4 inches, if odd add 5. For example if it measures 30", you are a 34" band size. If it measures 41", you are a 46" band size. Next measure your bust (fullest part of breast usually at nipples) find that measurement and subtract FIRST band size measurement. The cup size is inch difference between band and bra. An A cup is an 1 inch difference, B cup is 2" and etc. For example if you measured your bust as 40" and your band as 38" BEFORE adding 4 or 5" then you would be a B cup.
I am a 34C because my band size measurement is 30" (adding 4" gives me a 34) and my bust is 33". Cup size is C because (33" - 30" =3" difference)
With transwomen it can often be difficult because sometimes chests curve upwards in bone structure and may have a bigger difference in bust size to band size because of purely muscle and bones. Some transwoman with no visible breasts could be a 40B or 32D because of differences so always allow room for variables. If you need further help you can google how to measure bra size and use a measuring tape if you are not comfortable getting fitted by others. =)