a couple sources:
Jeffs Bronco Graveyard; Dennis Carpenter is starting to get more parts; LMC Truck, Mac's/Eckler's is even is getting more parts for these trucks in the last 2 years, etc. Often there is compatibility between broncos and F series.
I'd say go to a junkyard but since i grew up in Michigan, i know everything from 1979 dissolved into rust dust circa 1987 lol.