Hey D.,
I'm in the same boat as you with regard to bra size. When I was pregnant with #3, I overheard the sales clerk at Motherhood tell another customer about a woman here locally that distributes the Bravado nursing bras and they specialize in larger sizes. I asked about her and got her phone number and then very promptly lost it. My memory was so bad with this pregnancy that I never remembered to call back over there to get her number again.
I would imagine Motherhood would give you that information if you called over there. I use their sleeper bra tanks and they work pretty good at holding me up and are really comfortable as my breasts swell and shrink. I replace mine every few months as the elastic breaks down and buy a size smaller to get extra support. Not to mention they are really convenient for breastfeeding and work well underneath clothing.
Good luck and let me know if you're able to get that lady's number!