I have never found replacements for the OEM plastic covers. I'm going to have my belts re-webbed (by Seatbelt Planet, $140 ea.), then use vinyl paint to refresh the covers. It will cost me more to re-web, but I wanted to keep the original set up. They have 50 styles/colors to choose from.
https://www.seatbeltplanet.com/i-30498197-seat-belt-rewebbing.html
On my '79, I bought a kit from Seatbelt Planet. $275 for all 3 belts w/ hardware and covers. It uses an "L" bracket to mount the retractor to the floor. One thing I found they tend to lock more often while driving around than the OEM style.
https://www.seatbeltplanet.com/ag-1781674.1781684.1781693-1985-chevy-pickup-truck.html
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