So, let's say your truck is from the north east, and 31 years old. Then they become a "rust" part. I've had a really hard time locating any, I thought i found some at a salvage yard's online site, ordered them, and then got an email back saying "actually, we don't have those." Figures.
I need both front lowers, and the steering knuckles.
I've looked at a multitude of "gmpartsXYZ.com" sites, none of which have them showing as "discontinued". Jegs, Summit, LMC, Rock Auto, none of them show any aftermarket replacements either. Of course, you can find the 2WD ones no problem. I've got a 1990 K1500, so they'd be different than your one ton, but I'm in the same boat as you here.
I've reached out to Stephen Watson of Offroad Design to see if he has any leads. If i hear anything back, I'll post an update.
Wish i could post in the "Wanted" forum, but alas even paying my $20 doesn't grant me that privilege yet...
P.S. your K3500 is bad ass. If you strike out on the control arms, and have the cash and desire, you should look into a solid axle swap. If you plan on adding any lift, it might be worth it in terms of reliability.