Year, make and model of car and what type of tach are you working with?

Will one of these work?

MSD TACH ADAPTER 1964-1995 | CJ Pony Parts
Auto Meter 9117 Auto Meter Tach Adapter for DIS Ignition Systems
outside of that I would look for the oldest Ford performance dealer int he country and try them, someone like..

MPR: 100 years in the business
I also have some feelers out there, will let you know