I am string, a black nylon string for use with string linkage rotary valve mechanisms. String sells by the imperial yard, but is cut to longer lengths as desired. One yard (91cm) could easily restring six rotors.
If you're new to the idea, check some videos on the internet showing how to restring a rotary valve.
Also keep a lighter handy. If the end of the string frays and refuses to go through a key lever hole, heat the end of the string briefly with the lighter flame to prevent further fraying.