If they are the original leaf springs you should probably replace them. If they're not original to the car then you can just replace all the eye bushings and reuse them.
There are quite a few Mustang specialty suppliers (National Parts Depot, Glazier-Nolan, Mustangs Unlimited, etc.) that supply quality replacement parts. I replaced my leaf springs last year with the Grab-a-track 4 leaf units and am very satisfied with them.