Ok, so this might be a bummer but I have not found a good way around having to purchase at least two bras during a pregnancy/lactation. But I can offer suggestions about how I avoided buying like four!
When I am at my normal weight, I wear a 34B, not lactating. But I've been holding onto weight and have been 36C for a year, lactating. I go up to a D or DD right before birth/ after birth and for several months. Then they shrink slowly back down to C or B.
That is a huge range. And not only the cup, but the band changes to as ribs expand for the uterus.
So, I have borrowed bras from my mother, who is larger than me, I've bought cheap ones at maternity consignment stores, I've gotten cheapies at Target or layered my old sports bras if the were stretchy enough...I say don't buy a breastfeeding bra until the end. You won't need it for at least the first week as you recover and lie in bed with baby skin to skin or putter around the house. Then once you've lost some water weight and uterus, buy a nursing bra. It will last longer that way. One you May want during the first week is a nursing sleep bra that is very stretchy and not really supportive, but gives enough to walk around the house and hold nursing pads in place so you don't leave a trail of milk behind you whereever you go.
My fav brands are Bravado and LLL. At this point, underwires are not recommended for lactation. I don't think underwire is good at anytime and try to avoid it, but I still have one or two oldies hut goodies that have wire. Hope that helps!