It's a volume thing - the LS430 is a relatively rare car, air suspension is a relatively rare option (only available on the top two trip levels), and the replacement parts are only available through lexus. There isn't anyone who rebuilds them or sells them private label.

For the price of 1 air shock replaced at the dealer, you can get an independent mechanic to put the car on conventional suspension and it'll still ride good enough on smaller wheels.