I've been considering an air setup as well. My research lead me to this hybrid system by Platinum VIP; which I'm considering:
http://www.phantomsuspension.com/
Two other players are Air Runner (
http://www.airrunnersystems.com/air-suspension/) and Air Lift Performance (
http://www.airliftperformance.com/). I believe member 2theextreme has an Air Runner setup...