I have enjoyed the YES bra which has been mentioned in another post on this site. If you contact Christie via details posted elsewhere on this forum, she can give you a virtual fitting and she even sent me two sizes so I could try both and return one of them. They go up to very large sizes.
I have found that it's not an exact science though, estimating your bra size in advance. Some people are OK with only one cup size bigger than their normal size, but others' breasts balloon up to three cup sizes larger. I would advise buying only one or two (I have a couple that were fitted by "experts" but have been unwearable), write down details and keep handy so that you can despatch somebody to get another one, or a bigger size in case that's needed. Don't underestimate the fact that engorgment, although usually temporary in the first month, can be painful if your bra is too tight; you will rather go without a bra than have something constricting them if they are tender, but the weight of your breasts will definitely need support!
Those bra extenders that you might have used in pregnancy are good after birth, too; what ended up being my perfect bra (one I bought in the UK) was too tight on the band for only the first week or two after I gave birth, and it ended up being the only bra I could wear when I was engorged.