Really, you can't get non-metric stuff? In the US, every hardware
store carries both metric and non-metric. Somebody, somewhere, must
have one.

Check with your a large camera store. 1/4-20 is everywhere. It is the
standard for most tripods and cameras.

Otherwise, check with your local school and see if their technology
class has a die so you can cut the threats into another nut. That
might be a good school project for the teacher. I had threads cut into
an enlarger's lens holder that way.

I'd mail you one, but the damn shipping would be way more expensive
than the nut, which is like $.05 (about the cost of a stick of gum).

The closest you will have is about a 6.5mm plus a bunch of Locktite.

Good luck. Keep looking and you should find something.