As much as I dislike Wal-Mart, I got nursing camis there for like $15 and they have really good support. Not quite as good as I wish, but oh well. I was a 32D before I got pregnant, now I'm afraid to measure them. Haha. But their "medium" size cami works good for me, I live in these shirts. I do wish they were a little longer though. I've heard good things about Target's camis.
A friend gave me some motherhood brand ones that are full sling. I like them because they're really comfy and soft, but not as much support as the wal-mart ones, and they cost more money.