We are trying to "fix" an old kitchen range.  It's a 1977 Electric
Corning Counterange (flattop) Model R20JB.. We need the manuals that
came with the range and leads on where to buy cookware that came with
the range.

1965-The same year, Corning also introduced a "Counter-that-Cooks"
electric range made with a Pyroceram cooking surface, designed by
Corning staff designer Jerry E. Wright. Under the smooth glass surface
were four unseen heating elements. With it, Corning provided a set of
Cookmates, special pots with ground and polished bottoms to assure
perfect contact with the heat source. The product line was refined
into "3 plus 1" cooktops, which enabled ordinary pots and pans to be
used, and in 1976 was sold to Amana. By the 1990s, all major
manufacturers had a similar black "glass-top" range.
Looks like you should contact Amana..

ebay seems to have some of the cookmates from time to time. None are
currently available.