Cash in hand. Preferrably one for an old Eastman-Graflex / Folmer-Graflex with the metal handle, there were a bunch for sale recently of which I bought a few but the owner had no spares.
It would be a good idea to try to measure the holder for the exact size you need. Not all darkslides are precisely the same size even though you might expect them to be.. If you get one that is a bit larger than what you need, you can sand it down. If it's too small, then you are just out of luck.
bob.