Where should I buy a spare cm870 ?

[Chamberpains]

I personally wouldn't go High mileage used. You might find a lot of core parts junk because of stress cracking because of it's unknown extended use or abuse it may have. If I had the chance to have a drop in ready one sitting around I would take a chance on and search for a low mileage wreck with a guarantee from the salvage yard in case it's found to be cracked upon disassembly and tear it down myself to save on labor of paying your shop of choice labor. Or buy a guaranteed good block and crank assembly (short block). Then I would take all the pieces and parts that could be potentially salvaged and drive them personally to the shop of choice. Even if that meant a 1500 mile trip one way to Georgia or Pennsylvania to save on shipping costs, after all most of us are truck drivers, why not pay yourself to do the job at the best rate on the market!
Let the shop you trust get a brand new head and cut the block and assemble it to the advised specs and procedures on this site. That way you know it was assembled correctly and with the best parts available.
Then if you don't have the facility to lift the whole engine in and out, call a tow truck (couple hundred $$, relatively cheap) have everything towed to a shop to have it installed. or better yet go rent a fork lift for a day or 2 (couple hundred $$, still cheaper than paying a lot of shops) and do the swap yourself.
Doing it this way ensures that everything is up to your standards and you can take the time and care to make sure it's done right.
All warranties are parts only warranties thru Cummins, no labor included. Save the labor by doing it yourself and having it assembled by trust worthy shops (GA and PA) and have faith that Cummins makes damn good parts, they just don't put them together the greatest all the time. That's where the trust worthy shop comes in to play.
This is only my way of thinking cause I was bit quite hard by letting other shops do the work. Now I'm redo'ing what I paid some monkey to do once already.