The swap meet here at the bolt Craigslist, as has been mentioned.
Truck Trader, which is at your local newsstand/convenience store.
eBay - they come and go on there
look for farm equipment and estate auctions within driving distance from you.
Look up the websites or local online news sites for small towns and rural areas around... sometimes they might have a listing of something... or mention of an auction.
Just about every website that is like the bolt here, has a 'for sale' section... sometimes it is in the forum, sometimes it is a webpage.
searching for used GM truck parts You can check out the different ones there.
Enjoy,
-W