The location of the source of the mud is a closely held family secret, but I bet the OSI geolocating people could find it quickly based on this video[0]
It's not "really" a secret - anyone in the area of Palmyra who knows baseball probably knows of Jim, and knows almost exactly where it comes from. I think what keeps this secret not well known is that it isn't exactly a gold mine - there is only a single buyer for the mud, and the sales total to about $20k/yr, with a lot of manual labor collecting and filtering the mud. It's not worth it for anyone to try to scoop his mud source.
I think the interesting side to that perspective is if the mud could be used elsewhere. Sure it's only used in baseball now and that's an extremely small market (relatively speaking), but if other uses could be found for the mud, maybe things change.
Check out paragraph 3 of the Discussion section.
That's a pretty sweet side gig for him though. I have to imagine that he has access to MLB paraphernalia just by association with them. That alone would be cool, let alone the 20k.