the 3-15 is a mid 70s power king or economy tractor. they're all manual steering boxes. If it's not functioning properly i think you should be able to repair it. meaning bearing, collars, pins etc.. if it's sloppy it can be rebuilt.
if it's missing or the housing is broken, i would find a 1970's power king or economy tractor (any model) and adapt it to fit. mine is the economy tractor.
if you were really creative you could take any manual steering box off a car/truck but that would take additional work.