If you cannot find replacement springs for the rear, find a shop that builds leaf spring packs. They should give you a "cheat sheet" you can use to document the various measurements for the new springs. In general, rearched springs seem to sag not long after they are installed. Consider using airsprings or airbags on the rear axle. The coil springs in the front should be easy to replace so cost is minimal. Look up "Sumos". They might help a sagging set of springs similar to airbags. When installing airbags, keep the schrader/tubing separate. They can provide you with the ability to adjust the height of each side independently.