I have three super cheap nursing bras: two from Motherhood Maternity and one from Kohl's -- one fits now, one is a little big but doable, and one is a size too big. All can be adjusted up. All were on sale at the time.
The shop at the birthing center sells Medela bras and DH gets an employee discount (he works for the hospital that owns the birthing center), so I'm going to wait until I'm there to buy my "nice" nursing bra. I'd been told that I'd go up one more cup size again before the bean comes but that hasn't happened. I did go up during pg, but haven't in this last trimester.
My mother went from a 32B pre-pregnancy to the largest size they sold at the time, a 38D, when her milk came in (the bra was too small). But she went back down to something in between after a few days, so I'm hesitant to buy a bra when my milk first comes in, since it'll be too big after a few days/weeks, yes? And I might not even buy one before my milk comes in since I don't have a clue what will happen after.
I was a 32A (or AA even -- hell, I never wore bras pre-pg so who knows for sure) and am in a 36C now. I never really understood how you could buy a good fitting nursing bra while still pregnant, because how can you know for sure what your breasts are going to do?
Sorry that's no help at all, but I really have no clue how to know with certainty what size to buy ahead of time, thus the super cheap bras and I reckon I'll wait and see.