You have to order it directly from nachamichi parts, or a nachamichi dealer. Usually they try to force you to buy an "upgrade kit" and will not sell just the part you need.
They are slow and expensive in delivery.
That's what we put up with when we fix nachamichi-s.
There are also aftermarket parts distributed by companies such as Waldom. You have to measure the old part, and order the new one by size.
Or take it to the nach repair center and pay a lot...