First thing to do, is to get it up on a 4 post lift, and remove the lower part of each shock. Then measure each side of the car at the same point, preferably, in the wheel opening in direct center of the axel center point. That measurement will tell you if either side is sagging. If the measurement is between 1/4" and 1/2", it would be considered acceptable. If greater, then you need to have the springs removed, and either re-tempered, or replaced. A larger truck spring dealer should be able to handle the job. New springs are going to be about $1000, so I would go the re-tempering route if possible. A reputable spring dealer will be able to guide you best.
As for air shocks, I have seen them mostly on ambulances, but that doesn't mean that a hearse or combination car couldn't have them. Look your car over closely, and see if you can see where they were installed. Current production air shocks will cost you about $150 per pair, plus installation. They will not fix a weak spring, since they go up evenly. If you are starting off crooked, you will still be crooked until you have the spring issue addressed.