French thread freewheels can be found on ebay semi-regularly in varying condition. Ones with larger sprockets seem to go for more.

I sold an atom 14-26 french thread on ebay a while back. I wasn't sure of the threading, but a bidder told me a way to test it if you don't have an open hub around:

Standard British threading is the same for freewheels and bottom brackets - so if you have a spare non-drive side bottom bracket cup it should be able to easily thread into the freewheel. If you thread it in a little bit, you can usually take it out by hand with little force.

That should at least confirm if it's NOT standard threading. Based on manufacturer, you should be able to narrow it down to french.