It's real hard to find the truck motors. I'm sure you've noticed if you put "5.3" in the search you will get nothing just due to the software not recognizing it. I usually put Silverado engine, Tahoe motor, etc to see if there's one. Most people will put 5.3 or 5.3l in the ad and never put Lm7, Lq4 etc. Occasionally they put "vortec" in the ad, and it's not too hard to weed out the Gen I motors. Sometimes "5300"
I have occasionally run across the SSR engines (the aluminum 5.3l with LS6 heads.. Some other vehicles came with these engines too), sometimes for $700-800. I knew I should have bought that last one! Oh well
Just remember, you're better off to buy a low mile 6 liter engine for $1500 than to get a free 6 liter that needs rebuilt!