Cup and band size is relative. If you go down a band size you normally increase a cup size to maintain the same volume. For example, 34D-32E-30F-28FF-26G. If you go up band sizes you decrease the cup size. I measure 25" underbust. My nearest band size without adding inches is 26. With my bust measurement I'm a 26GG. 26 bands are non existent except for special order so I have to go to a 28 band. Going up band size I'm 28G, 30FF, 32F 34E, etc. I used UK sizing (American bra sizing is so screwed up). It took me a long time to understand this.
Band sizing also has to do with the amount of "squish" you have. I have no squish, there is a 1/2" difference between my break the tape measurement and my snug measurement. Yesterday I sat next to a woman at the theatre who probably "thought" she was a 38-40 band. She had lots of squish and her band was riding up between her shoulder blades. Classic sign of a too large band. On the subreddit /A Bra That Fits there isn't a day that goes buy where some poster is saying "OMG I just measured myself and I came up with a 26, 28, 30 band. Is this possible?"
I don't know how weight loss affects this, I'm guessing it depends on where you lose the weight from.
I'm finding my bras on Amazon.com. Other online companies carry the odd sizes too.