Replacing a cracked MacBook Retina Display with third-party hardware?
Solution 1:
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.