Don't buy anything until after your milk comes in. Get some maternity/nursing sleep bras which have stretch in them - pretty much like glorified crop tops but with clips so you can drop a side to feed. They'll do you until you can get properly measured.
The last thing you want is an ill fitting bra on engorged boobs, trust me; ended up with mastitis on day 14
I ordered a load from an online company in different sizes as soon as milk had come in and sent the ones That didn't fit back. You could get a couple of soft crop top ones just to have beforehand, but you will get time to order some in the first couple of weeks
I had to get underwired ones as the non wired ones gave me a mono boob which was really uncomfortable
I'd recommend bravado bras especially if your not sure about sizing because their bras have flexible sizing. I used them when I had my DS and I found them perfect. I still use them now when I can't be bothered with underwire and he's now 19months! I got them from here: www.nursingbra-shop.co.uk/ I found them to have the best prices. But shop round cos you could find a bargain.
I have just got one of the carriwell seamless nursing bras for the first couple of weeks. They are comfy, quick drying after a wash and ok for sleeping in too. Not brilliantly supportive but at that stage, I just need something to keep my boobs off the floor and to stick the breast pads to. Once my milk has settled down I'll go to bravissimo and get a couple of proper nursing bras.