These are like hen's teeth, but I see them come up once in a while on the auction sites. With a part number from Gary, you can search places like rearcounter.com or greensalescompany.com
I've had good results repairing them with fiberglass mesh "painted" over with plastic epoxy on the rear of the trim parts, both for the pillar and door trim. I clean and rough both sides of the crack (and even full breaks), lay down about an inch on both sides with the fiberglass mesh I use for drywall seams, and then then lay the epoxy over that, sometimes with a piece of aluminum flat bar as a backplate.
Although I've used JB Weld plastic epoxy in a pinch, I've found the Polyvance products a bit easier to use and is of excellent quality.
Sonoma County,CA
1982 F150 Flareside XLS
NP435 4x4
351W Motorcraft 2150