This should be fairly simple as leaf springs can be custom made pretty easily. Some place like Calvert Racing or
www.eatonsprings.com can take the dimensions of your existing springs and desired drop and spring rate and make you a custom set from scratch.
Leaf springs have been in use for hundreds of years and are still widely used so finding someone who remakes them or services them shouldn't be a tough order. Maybe even in your local area.
If those don't work, see what all the F-Body guys are using and contact those companies or even consider adapting F-Body parts since aftermarket stuff for them is a little easier to source than AMC stuff but i really think you'll be surprised how easy you find someone willing to make a custom set.