Measure up threads, and get a set made at any decent place with a lathe and a mill to match your spindle threads. Mine are 2 1/4in 8tpi witworth threads, pitch angle on wheels (anodized red and blue)
The chance with buying from somewhere and them not fitting is pretty big.
Lots of retired guys with a lathe in their shed willing to make you a set. Look on ebay for Lathe & mill services.
Morten