I've done this on a 98 Lincoln Town Car, very easy to do. I got the Springs & Shocks on ebay. It cost me about $50 for Springs and $20 for Shocks.
Some Info here>> http://www.google.com/search?hl=en&q...&aqi=&aql=&oq=
shop around for the best price.
Car rode good after swap to me. Sat slightly higher in rear, maybe an inch. It's not gonna be the floating cloud ride the Air Bags provided but, it's cheap.
If you want smoother/softer throw a few kids in the trunk.
Spring kit came with instructions showing which wire to unplug in trunk to disable Air Pump.