Sunn Beta Lead 100, replacement pots

Hi guys.
I have a Sunn Beta Lead, and I'm wondering where to get these particular potenciometers for this amp. For example, the "drive" pots are 1 Mohm dual gang, with the 3 pins of each gang assembled 180 degrees opposite of each other. And the rest of them are 100k pots with a 4th pin to each one. They are beginning to fail on location, and make a crackling noise, and I just cleaned them with contact cleaning spray.
Any ideas where to buy them? Or a complete set of new ones (13 pots in total)?
 
Google reveals a new company using the Sunn name and that Fender once owned the rights.
However, I would be surprised if all locations made use of the 4th tap.
Maybe they used the same pots throughout whether they needed the tap or not?
Trace it and see.

Centre tapped potentiometers do exist though they may not match exact dimensions.