Just a long shot, but you can try any Jaguar, Benz or BMW specialists, maybe they have used units taken out of owners fed up with replacing their air suspension!
My dad's Daimler had air suspension, converted to springs after replacing it once (costs RM9,000!) and then it broke down again.
Or you can also try google for "Bridgestone Praxis", but the air spring application is rather limited at this moment.