Rear wheel and freewheel hub with disc brake mount for $48
Wheel 20x1.75 Rr FW/Disc BK/BK - Choppers U.S., LLC
Complete rear disc brake kit with caliper and rotor that will mount on above hub for $36.
Disc Brake/Rotor Mech - Rear - Choppers U.S., LLC
If it's for safety, it's a well-spent $80
Also, here's a $9 mounting tab that can be welded to the frame for the caliper.
Raw Disc Caliper Tab - Choppers U.S., LLC