You might try eBay... things pop up from time to time. There's one guy there... Cliff Gravel out in Arizona, who seems to have lots of old timey stuff. I first got Roland Winters Charlie Chans from him (he has most)... and a long lost Sherlock Holmes/ Arthur Wonter, The Sleeping Cardinal.
You can get to Cliff's listings by typing in, say, Charlie Chan, and he will pop up as one of those who has them... and you could then look and see if he has the Mantan Moreland's... could email him too, and ask if he knows of who might.