Most reliable way to get large amounts of plutonium is robot farming, either with a captive eyebot (close it in a car so it can’t get away) or with low computer skill and analyzers in labs (which drop manhacks when you fail a certain way).
Otherwise, simply butchering every zcientist, shocker, technician, bioperator, etc, everywhere you go will give you a pretty good pile of power CBMs (which, yes, give you more power if you can’t them into the Mk IIs instead). My late game characters generally have more plutonium lying around than I bother to use for anything. You need 8 plutonium cells per thousand power… so 4000 plutonium. Hmm, that is a good bit. Not sure I’ve ever had QUITE that much, though I’m been at least a quarter of the way there (not counting all the power already installed…).
For power converters, arcades are very good, and school computer labs aren’t shabby, either. The average Lab has several analyzer rooms (which 4 computers in them, counting the live one) and many observations rooms (same). Don’t forget to disassemble the terminals after you hack (or jackhammer) into barracks and prisoner containment rooms (3 each). Power substations and such are also pretty good (unless it’s changed fairly recently, you can disassemble most of the equipment for useful things).