Replacing a cracked MacBook Retina Display with third-party hardware?
Apple doesn't sell replacement parts to users, only to their authorized resellers and service centers. However, some used Apple screens are available in the aftermarket, as, for instance from ifixit.com. 

I think your choices are:

A) Have Apple or an authorized shop replace the cracked screen;

B) Buy a used screen and install it yourself;

C) Buy a used MacBook (perhaps you could find one with broken electronics, or a bad keyboard or inoperative logic board) and take your chances that its screen will be good;

D) Find a non-authorized repair shop that can replace the screen (thus offloading the "where to get the part" challenge onto the shop, along with the risk that the "new" screen won't work); or 

E) recycle your MacBook and replace it.