I've always worn a 40DD because that's usually the biggest size stores carry (if they do carry DDD, it's almost always in an old grandma style that I can't bring myself to wear).

I just tried on my first bra from Lane Bryant, ordered with their sizing guidelines in a 40H, and wow! I feel like a new woman. So expensive...but so supportive. *swoon* Breastfeeding may make me feel like I'm going to trip over my droopy breasts when I get up in in the morning, but at least I can put on this magic garment and appear somewhat normal.