you can get a used one from wrecker, but may not be what your looking for. reman'd ones can be found at any NAPA or the like, and you could probaby get a new one there too.
you can search upgrades in this dept too on here, durango or dakota boxes I think, aswell as YJ, like you said, but they entail a bit of work.
and I think your talking about using the AC condensor as a cooling res for PS fluid? If so, yes it can be done if you no longer use your AC, you can also find small cooler used on some tow equiped vehicles, or again buy new at NAPA.