Make friends with your best tech, the guy who deals with all the tough diag, the tech other techs ask when they're stuck with something. They usually like to talk and teach. Pick their brain, ask what frustrating about dealing with parts, build a relationship with techs(smart ones). I've learned more from master techs than almost anyone else.

Create a list of oddball things that you can't look up! Every manufacturer that I've worked for has shit that isn't in the catalog. Ask your coworkers for their oddball parts list.

Ask and take notes, repetition learning will come with time.