May have to get it as part of an entire kit.
However, since its a part of the self-adjuster - that may also be available as a seperate kit.
From '63-'68 the brake systems are basically unchanged. A slightly different shoe was used in '68 that's backward compatible.
In '69 the self-adjuster system and shoes were changed again.
So what you want to look for is the 63 - '68 adjuster, and self-adjuster hardware and spring.
FYI Some info about the 'new' servo contact brake system
Servo-Contact Brake Service (Session 187) from the Master Technician's Service Conference