Crank handles for the 645 Super/Pro/Pro-TL are very tough to find, so much so you'd have better luck looking for a junker body with a crank--good luck!
Frankly I got a power winder to improve my Super's awful brick-like ergonomics. Would lose a minimum of 3-4 frames/roll from goof shutter releases while fumbling with it on the street or in the studio. The power winders are noisy but necessary for handheld shooting.
Thank you. That's my luck. I can go to Auto Zone and ask for an alternator for an '89 Chevy and they'll tell me," Man, good luck on finding one of those."
I'd just start prowling around ebay. You might find just the crank, but more likely you'll see one on a beater body that works perfectly well. You can save your search and get notifications from ebay if something should pop up.
Not that I have reason to know about this, of course.