For concours-trailered, you wouldn't need NOS or original for the battery cables, the marti auto reproductions are acceptable for judging. You may get sticker shock if you price just the positive cable in NOS condition. The negative cable is very difficult to find, even used. Starter cables are probably the easiest ones, but condition is a factor.
If you absolutely must have NOS battery cables, Bob Perkins may be willing to sell a positive cable.