I dont know in the states but the positron cable is extinct! shimano dont make them anymore

so the only ones you find are NOS.

the good-
positron was the first in history of indexed deraileurs
too bad shimano made that feature on there bottom line

the bad-
bust a cable and you are S.O.L
I save the cables,
bikes with positron in the dump I leave there.

my opinion is you did the right thing, upgrading to a repairable system.
so does this answer your question?