Coupler offered on eBay
I just stumbled across a Tektronix shaft coupler on eBay and recall someone on our group was seeking a coupler.
I have no connection to the seller:
Good luck if you need it.
Tektronix 376-0050-00 flexible coupler for 0.087/0.127" diameter shaft TM500
eBay item number: 153936604515