Inland shifter gate spring replacement...

1966-1968 INLAND SHIFT HANDLE REVERSE CABLE/SPRING - Mopar A833 4-Speed Transmission & Component Specialists
If you still want to take a stab at it yourself, you could get some pretty close approximations of size by doing some comparison measurements on your cable and compare to the picture in the above link.
You could also call Brewer's, since they rebuild and sell rebuilt Inland mechanisms it would be reasonable to assume they have the springs- even though they're not individually listed in their online catalog.