I would hold out as long as you can on buying bras and then get professionally fitted.
I bought my Pregnancy BF underwire bras online..Frayer maternity and very comfortable. I went from a 10c to 10DD-E.
When you milk first comes in you may find you may not be able to wear a bra at all and wear something like a BF singlet that has like a shelf bra. When it settles down you should be able to go to wearing a BF bra. It was around 8-10days after giving birth that I started to wear a BF bra.