Hello MummyTurnip. This is how I do it. I've breastfed three children and I am currently expecting a fourth.
I am normally a 34F or FF.
I normally go up one back size (i.e. to a 36) and one or two cup sizes. So I think I will need a 36G or thereabouts.
I think this is quite typical - when I have been measured, this is what has been recommended.
You could therefore probably risk buying a 36D. You can buy extension pieces and silicon shoulder straps from www.bras4mums.co.uk to adapt existing bras to make them more comfortable.
The National Childbirth Trust sell very good feeding bras and are very good about giving advice as well. There are sexier ones on the market, but the NCT's bras are designed by mothers for mothers.
www.nctsales.co.uk/prodlist.asp?cat=250&scat=253&parent=250
Official advice from the National Childbirth Trust:
The ideal time for a pregnant mother to be fitted for a breastfeeding bra is at 36 weeks. By this time your baby's head will have engaged and your ribcage will have expanded by 1"-2" 1-5cm. The ribcage will return to its normal proportions within a few weeks. nct extention pieces can help with these variations in size.
During pregnancy and following birth, the breasts usually enlarge and increase in weight, most pregnant mothers expand by one to two cup sizes - an increase of between 2"-4" / 5-10cm.
When purchasing a breastfeeding bra it is essential to leave room in the cup (especially at the top) to allow the breasts to round out and for the milk to come. You should be able to slide the flattened hand inside the top of the bra cup.
Breasts should be fully enclosed and supported in the cups- you are not looking for a pushed up cleavage effect, in, fact, wearing an underwired bra whilst breastfeeding can cause blockages in the milk ducts.
As with all bras, the strap around the body should be horizontal, snug, firm and comfortable (it should not ride up your back or able to move around).
(Information from this link)
www.nctsales.co.uk/nctbra_info.asp?cat=97&scat=97&parent=250&type=1
Good luck with the c-section!