If you begin your search on eBay...
Buy and sell electronics, cars, fashion apparel, collectibles, sporting goods, digital cameras, baby items, coupons, and everything else on eBay, the world's online marketplace
www.ebay.com
...it will give you chance to check out the ones that look promising from the ones you should
'Shine On' and absolutely avoid getting. Here are some additional suggestions:
(1) Avoid anything with over 160,000 Miles. The Atlas Engine
CAN run as long as 300,000 Miles... but getting beyond the 100,000 to 160,000 Mileage realm means there won't be but a few more years left on the engine. Anything Older and you are just asking to inherit someone else's problems.
(2) Follow your "Due Diligence" by confirming that the Vehicle was recently towed into the Salvage Yard after an Accident and not just dragged off of somebody's Vacant Lot after sitting there for over a decade. Request additional images and specs from the Invoice if you can get them to email you this data.
(3) LQK Motors sells GM LL8 Engines for a Higher Dollar... but they may include a Limited Warranty.
(4) Keep your Motor choices within the range of your present (2002-2004) PCM if possible. The Later Model Engines will NOT play well in the 2002 First Year of Manufacture SUVs.
(5) If you have a place to work that you can park ANOTHER GMT-360...it might be cheaper to just find another, low mileage SUV that will cost just about as much to get as a Used Engine and allow you to skip having to crawl around to R&R or transport the Motor out of a Bone Yard. Besides THAT... it provides all those additional Parts that may just come in handy.
(6) If you have access to a Pick Up Truck... Your options for doing a nationwide search to find the Very Best Deals becomes possible if the Seller(s) will ship to a Local Freight Yard at a Flat Rate. THIS opens up ALL the doors to you so that you can afford to be *Picky* and find a Motor worthy all of this trouble.
To see what this activity looks like when swapping out things wholesale between one GMT360 and another... look up "The Daninator" Youtube Channel for a whole series of such R&R and Swap Videos.