PST still seems to carry them as part of their rebuild kit. https://p-s-t.com/i-23162823-original-performance-super-front-end-kit.html#!year%3D1971%7C%7Cmake%3DPLYMOUTH%7C%7Cmodel%3DFURY%20III
at the time the whole kit from them was cheaper than 2 ball joints from rare parts. might still be.