Finding just the knob will be very difficult. Once they come off, they normally do not go back on. You're better off finding a large muffler knob, slightly drilling it out and tapping it onto the existing rod. I don't recall the actual drill bit size I used, but you don't have to open it up much. In the attached pictures of my repaired Zoomatic, you can still the threaded portion of the knob. Hope this helps.

-Mark