If you can't find one out there, I would try to call Mesa as they probably have some kicking around there. I've used them for parts (pots, faders, switches, LED holders, etc...) before and they are priced pretty reasonable and a big bonus is that you don't have to waste time researching to find the right replacement part.
There are some type 8 - 2 spring reverb pans out there there match the impedance you need, but, yours is a type 9 - 3 spring medium decay.
Good luck. - Pat.