Yes, you should be able to buy just the mirror itself. Talk to the service department at your local dealership, Give them a call, but, do ask for the service department. Usually you put tape all over the cracked mirror, gently break the old mirror, The pieces come off with the tape. The new mirror has an adhesive backing.
DONT BREAK ANYTHING TILL YOU TALK TO THE SERVICE DEPARTMENT. They may or may not be able to get that piece.