Welcome.
I have two '86 K30 regular cab duallies. One of 'em has been sitting at my Cousin's body shop, for the last six months, and was damaged while there. So, I'm kind of in the same boat as you.
Here's what I know...
As far as I've been able to find, there are no after market replacements. I
have had fenders repaired in the past. (*In fact the fender that was damaged, on the truck currently at the shop, was repaired once before.
)
As close as 4-5 years ago, entire beds (in decent shape) were still pretty readily-available on Craigslist. Unfortunately, Square Bodies have since gone "Barrett-Jackson" and "Mecum". So, when you have folks with more dollars than sense take an interest in your hobby, you can pretty-much bet that they'll do their best to ruin the whole experience for everyone.
Despite that, my recommendation would be to keep an eye out on Craigslist and the like. Regardless of the recent explosion of interest, I still feel that might be your best bet. Happy hunting.