‘72 Plymouth Duster master Cylinder and Booster

If it's the 7" dual diaphragm booster, you ain't gonna find a new one. Rebuilders are your only option, like Booster Dewey or Harmon.
Vacuum Brake Boosters | Power Brake Booster Exchange, Lynwood
Home - Harmon Classic Brakes
Doctor Diff also offers one that, although not an original, should be a bolt-on. But I would contact him first to make sure it will fit your brackets.
Bendix Style Dual Diaphragm Booster for Mopars