First off, welcome to the site. Lots of info here. Just look and ask.
I guess it depends on what you are willing to spend or your experience level. If you search Craiglslist and find something local, make sure you can hear it run before purchasing. If you have to pull it yourself, this may get you a cheaper priced engine than if it was pulled by someone else. By pulling it yourself, you can also be meticulous about pulling the harness and associated pieces you will need to complete the swap into your older truck. Plus you may be able to wheel and deal for needed parts with a private owner compared to a yard.
Engines already pulled are going to cost more and without the benefit of hearing it run. I would think you are also at the mercy of the yard that pulled it for accessories needed for the swap.
Personally, I would look for a 6.0 liter. Either an LQ4 or LQ9. No replacement for displacement. If you have the opportunity to listen to the engine prior to removal, and check it's oil pressure, I wouldn't hesitate buying it if it sounded good. That is, of course, it has the exhaust system intact so you can hear any potential noises(knocks or raps).
My personal experience is with an engine purchased locally off Craigslist. It was still in the truck with exhaust intact and 144,000 miles on the clock. After sorting out some wiring issues to get the fuel pump to run, it fired right up and settled into a nice smooth idle even after sitting for over a year. I wheeled and dealed with the owner who had it listed for 9 or so months on CL. He wanted $500 for the engine. I got the engine, harness, computer, entire exhaust system, transfer case with blown 4L80 trans and fuel pump for $600. Granted, I spent two afterrnoons after work getting the engine unhooked and cutting out the harness carefully. But I feel I got a great deal. He also sold me the 4L60E trans out of a '99 Escalade for $150.
After selling my old 5.7 liter Gen I engine for $150 and the blown 4L80E along with the accompanying transfer case for $100, my total out of pocket expenses for the engine was $500. Tack on $300 for incidentals to get the new engine up and running, and that gets me to the total of $800 for the entire swap.
As you can see, deals can be had if time is on your side.
Hopefully your search goes well. Good luck!!!
'93 Chevy 1500 4X4 ECLB with a '99 Silverado LQ4 6.0 and '99 Escalade 4L60E. SOLD IT!!
New toy: 2001 2500 HD 4X4 Extended Cab Long Bed
John