Kelderman's has a pretty good complete kit from what I understand. But the guys who run them on trucks like mine (first-gen Cummins) hate them, and say the stock springs rode better.
Cost is probably the main reason that they aren't used in most cars. That and the auto companies have no respect for people who actually look their vehicles over and do maintanance themselves. The diesel truck resource forums had some Dodge truck engineers on and all they could say to the questions about locking hubs and servicable front bearings, versus the central disconnect and unit bearings currently in place, was "why, when what we have works, would you want something like that?".
Toyota uses air suspension on their big SUV. I think the grand Cherokee might, too, as you can vary its ride height. I think GM used some sort of magnetic set-up in some Buick and Oldes models.