Buy it before you deliver, roughly in third tri. The size is relevant to your nipple size, not related to the breast size. If you are not sure what size you should buy, go to a shop to get your nipple measured (e.g. Mums and Babes at Wheelock have a gauge to measure), if you find the products too expensive you don't have to buy there, but the key thing is to get yourself measured so you buy the right size.
I recommend Spectra or Cimilre, can buy from Qoo10. For efficiencies, buy a double electric breast pump.