Reading about using radio dial cord for the Simplex roll tracker
brought back some memories.  During my engineering career I worked for
Magnavox and was involved in the design of several radios which used
dial cord.

We got it from several companies, one of which was Ashaway Line and
Twine.  I just Googled them and they appear to still be in business.
In the past I was able to get small sample spools of their product and
I suppose that one could still do that.  I recently rebuilt a Simplex
mechanical tracker and I was able to use some of the cord I had left
over as a sample.

Pat DeWitt - in snowless Indiana