I feel kinda stupid saying I 1/8 inch threaded are rare....there is one on my fork wall.
Came off a Diamondback mtb. So ya, they exist..right under your nose!
I'd send it but i'm in Canada, and really -it's not a super nice fork.
Look at any AL frame mtb's -older with Rigid forks. The larger AL tubing\headtube are more likely to be 1 1/8 inch. 1 inch I relate to steel frames.