I've also tried looking for this up here without success. Thinnest I've found is 3m fine line tape. But it's only 1/16" wide instead of the 1/64 we need.

Still looking too.

Came across a trade name used by draftsmen: Letraline. smallest I found was .4mm (point 4 mm). Have no idea how that compares in inches though.

1/64" is .015" and 4 mm is a smidgen over 1/8" (.125"). Hmm.