While searching the web for my wish list of items on Chummy I came across this page for we owners of the GMC P-30 chassis. I am not sure how realiable it is and maybe those of you with more technical expertise can vouch for it, or not. Here is the opening paragragh:
"There are many parts for your GMC that can be obtained very inexpensively by a trip or two to your local junkyard. Your savings can be significant, depending on the items you select and the amount of clean-up or repair you are willing to do. Sometimes, depending on the circumstances, your trip can mean the difference between getting to your destination in a timely manner or sitting in a repair shop or parking lot waiting for the part to be shipped."
He then goes on in a rather orderly fashion to list the parts which crossover from a commonly found vehicle to our chassis.
here is the wb site: http://www.gmcws.org/Tech/junkyardgems/