Frank,
I bought the caliper return springs and the rest of my brake parts (except grease) from Fleet Pride Truck Parts in Phoenix. They were in stock. Fleet Pride is a national chain and they may have a location near you.
I bought a case of clay based grease from an oil field lubrication specially supplier when I lived in Houston years ago, so I have no needed any more yet. If needed, I can dig up their name.
In case anyone else needs the part numbers for the Marquis Meritor ADB brake system, they are below.
Meritor brake part numbers for 2000 Marquis
Rotors; 3218K 167
Pads; S2R7017173
Helper springs; Kit 15018
Gerald