You'll need to take it to a restoration-friendly body shop. Most collision shops are not going to want to have anything to do with the job.
The next problem you're going to have is that (surprise!) you can't buy new hinges. Most likely, if it's sagging that badly, it's going to need some welding and redrilling on the hinges themselves in addition to the hinge pins and bushings. When door closing problems are let go that long, the pins usually cut into the hinge stampings, and while you CAN just put pins and bushings in, it's a short-lived band-aid fix. Get a new striker, too.