Nursing bras should be bought 2-4 weeks before the EDD.
Always get yourself measured (or measure yourself) for a nursing bra as the size is determined 1 size up the current cup size and one size down the current bustline. This accomodates for the milk-swollen breasts and the shrunken abdomen after giving birth.
Make sure that the nursing bra is in a simple design rather than a fancy, netted design. Go for netted or lacy designs only if they have a cotton lining underneath. Although it may look very pretty now, it can be really painful when tender nipples get entangled in the net/lace.