Where can I get the proper dust cap for my front wheel hub?

I lost a dust cap to one of the front hubs. I tried two from NAPA, one generic Chrysler one, then they ordered me one that was supposed to be "correct" for 1970 duster, neither fit correctly. Anyone have a part number for one that works? Or a link to one online? 1970 duster, /6, drum brakes.