Nursing Bras

I'm 31 weeks pregnant. I'm wondering about buying nursing bras. Right now the 4 bras I'm wearing all are all 40 C (though some fit better than others). When should I buy a nursing bra? I've looked at Bravado on-line - does anyone know what size I'd be in their bras? I've also looked at an Olga nursing bra. Olga is my usual bra (pre-pregnancy I was a 36C ) although I have a 40C now that fits well. Olga bras run smaller than most other brands (I was told this in Macy's lingere department) and I find both now and pre-pregnancy I can wear C cups that are very stretchy, if I try to wear a padded or less stretch C cup, the cup is too big and crinkles. Or should I wait a little longer, go to Motherhood and have them help me figure out what size to buy (even if I don't buy there).

I have bid on 2 Motherhood nursing shirts on eBay size large (I don't think it matters if they wind up being loose - I am more comforatable in the large Motherhood button down shirts now - my one medium one is too tight in the chest).

Any advice is welcome. LJ people have been very helpful so far.