I've found that Motherhood Maternity carries a good selection of nursing bras. My advice would be to buy one right away - about a size bigger than you normally are - and then wait until your milk comes in and your supply has leveled out a bit (about 6 weeks) to buy anymore. I found that I was a full D cup at first, but now I'm mostly a C cup (pre-pregnancy I was a B cup).

I searched high and low and wasted a lot of money on uncomfortable, poor fitting bras and concluded the was no such thing as a good fitting nursing bra. Then finally on my 3rd baby I discovered Bravado - LOVE LOVE LOVE them! Their website is bravadodesigns.com, and they are well worth the money.

Oh, and their website gives some direction in finding your correct size.

i also liked my motherhood maternity ones- but I was a b cup before. I have heard if you're bigger busted to begin with, they're not so great, so I suppose it's a matter of what size you were before.
I recommend ordering a few of the "sleeping bras" that are basically more comfy sports bras. I think they come in S-M-L. I have about 3 of those I just lived in and rotated through by the first 4-6 weeks like Mel said. Then once my supply evened out a bit more, I ordered a few of they regular type styles. I think my base size went up by 1 (instead of being a 36 it's a 38) and the cup is C cup now instead of B) So if you want to order online, I would say go up by 1 size all the way around.
Find out ahead of time about the return policy, and then even order a few different sizes to see what you like. Good luck!

Ok, this is my FAVORITE bra! its at target so you can try it on, I got a bigger size than i was before. http://www.target.com/Gilligan-OMalley-Nursing-Sleep-Bra/dp/B000KKX3FE/180-0072852-4873942?ie=UTF8&index=target&rh=k%3Anursing&page=1

Also I love these tanks. I really only wear the bra at night and I wear the tank during the day under regular shirts that way when I nurse my stomach is covered and really you cant even tell the you are nursing because you are totally covered. http://www.target.com/Gilligan-OMalley-Nursing-Tank-Ebony/dp/B000KLXGNC/180-0072852-4873942?ie=UTF8&index=target&rh=k%3Anursing&page=1 I am normally a 34c and I got a large tank. also i got a 36 d in the bra. good luck! congrats on b/f!!!