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Instead of "knob" you mean a speed crank. You'll have better luck searching using "speed crank". There is one on eBay as I type this.
https://www.ebay.com/itm/New-ARRI-Follow-Focus-Speed-Crank-K2-32730-0/263552502944?hash=item3d5cf40ca0:g:t68AAOSwQFJaqzx9