I bagged a full set from a guy who was parting out a longbed GMC from California. He was trying to sell the whole truck, but when I offered him about half of his price just for the trim, he agreed to take it off and ship it. It was in phenomenal condition. My truck is a shortbed so the rear strips I had shortened and tig welded up the ends again. They came out well.
Before I found them, i had thoughts of taking some of the later Chevy trim and having it altered to recreate the GMC 60/61 trim. It's doable, for enough money and effort.