The Dynaco balance pot is made of unobtainium. It was a custom pot with a zero-resistance track on opposite halves of the two sections; a log-taper track on the other half of one section; and an antilog-taper track on the other half of the other section.
The only way to get an actual exact replacement is to scrap one out of another PAS or an SCA-35. There are even two types: the smooth-shaft type used on the PAS-2x and PAS-3x, and the splined-shaft type used on everything else.
It is possible that you might find a usable substitute by scrapping out some other piece of tube equipment, such as a phonograph or a console that is being junked.
Some modifiers remove the stereo-mono-blend switch from their PAS. They can install two volume controls, one above the other, to replace the existing volume and balance controls. Others who have had failed balance controls have installed a second volume control in one channel only (usually the right channel), in the position of the balance control; and a fixed voltage-divider attenuator in the other channel. That way, at least they can trim up the volume between the two channels.