Dear John and others,  I have heard a number of piano repairers using
radio dial cord for various string requirements on reproducing and
player pianos, especially for the return mechanism.  But I haven't been
able to locate a supply.

I tried quite a number of Radio Shack stores, all of whom said that
they stopped carrying it about a year and a half ago.  There is
something similar called diamond string, which is used in more
expensive wind chimes, but I haven't found a source for that either.

Right now I am using a sample of something called Goliath
braid, which was given to me by a local piano technician, and which is
quite strong and doesn't stretch, but it is waxed slightly, which means
that knots tend to loosen and eventually come undone.

Vincent Morgan