Hi N.,
I would wait until your milk comes in. You really can't tell before then. I made the mistake of prematurely purchasing nursing bras the first time. In my last trimester of my first pregnancy I was a 38C and after my milk came in I was a DD. (I bought D)
With my second pregnancy I was a D when my milk came in, and with my third I was back to a DD. You just never know. The breasts take such a beating from pregnancy and nursing. Right now I've been nursing for 9 months with my third baby and my breasts are clearly two different sizes. I pray they go back to normal, whatever normal is, I don't even think I remember anymore.
I too purchased my nursing bras from Walmart or Target. They both have a great assortment of types and colors. (Target has more options) I was never fond of the underwire nursing bras. I was told by my laction consultant that they can cause blocked ducts and infections. Target has great tank nursing bras. I loved wearing those around the house. They also have nursing nightgowns which was great in the hospital when family/friends would visit. I didn't have to worry about exposing myself when nursing in the hospital.
Good Luck!