1 1/8" MTB frames should be super easy to find, it doesn't matter if they were threaded or threadless when new if replacing the fork.
1" had a short life time for MTB frames (as long as you not looking at BSO or entry level bikes), although far longer than 1 1/8" threaded, which was last seen in 1995, 1994 was the last year for all but a few holdouts, look at you Marin, almost anything older than 1993 should be 1 1/8", a few exceptions from experience are Bontrager and Univega (Steel, not Alu), sure there will be a few more, but these are not the norm.