I don't think anyone sells them assembled - in part because they don't come with the lower seat and Bilstein doesn't sell a lower seat for this application. It's frustrating, I was even trying to help Adventure Driven Design (4wd outfitter in AZ) a couple years ago with tracking down the necessary parts to sell assembled 5100's and it's just not practical because nobody sells a directly compatible spring seat a la carte.
It's possible to use ProComp 63065 collars as a lower spring seat, sort of... but they don't position the rim the same distance from the machined rings in the Bilstein shock as stock lower spring seats so you would have to do a lot of measuring and math to get the right ride height.
6112's would be easier since they come with springs and seats, the only hard part is making sure you select the correct ride height. Using one coil spring for multiple truck weights means that final ride height is as dependent on your truck's weight as it is on which ring you select.
Your best bet might be to just get standard not-assembled shocks and then buy a nicer spring compressor so that the assembly process is less of a pain in the dingus. For example
this kind is considerably nicer than the scary rental tool at Autozone, makes the job pretty easy. Do not use power tools to compress them, just lubricate the threads before you get to work and you can dial down the springs pretty easily and with a lot less fear factor than the rental tools.