I'm not usually on S&B threads, but having lost a considerable amount of weight, my back size has dropped from 36" to 27" (and may drop further). I hope the wisdom of MN can point me in the direction of a bra shop or supplier who sells 26GG (or thereabouts) as my poor neglected norks are heading inexorably southwards in my current 30Gs. I found corsets that do the trick, but these are not exactly a suitable everyday option.
I don't know if this is any help, but I recently bought a Debenhams Gorgeous bra. I'm normally always a 32, sometimes even a 32 is not tight enough, but with all the Debenhams Gorgeous bras I tried on, I needed to go up a back size and down a cup size to get a good fit. They go down to a 28 back, so if those come up small, they might be an option for you?
mirai There is a bra guide on here already. It could do with a little more info regarding fit and shape but I don't think I write well enough to offer my services. I nomiate StatisticallyChallenged or HazelNutt