If you are unable to locate a factory made replacement at a reasonable cost, you could
find another tail stock assembly and have a riser block made to hold the other tail stock
at the correct height on the lathe ways. The critical aspect is getting the tail stock centered
on the lathe ways so it is perfectly aligned on the machine. Many wood turners have modified
smaller lathes with riser blocks under the head stock and tail stock to increase the diameter
of items to be turned on the machine. Ebay has a number of listings for lathe tail stocks for
various sized machines.
The other possibility would be to find another tail stock about the same size for a 12" machine
you could take measurements from the tail stock and have a machine shop grind the bottom
until it matches up to your One Way dimensions.
Hopefully One Way will work with you and provide a factory made tail stock at cost.