So i have been searching around for the black glass marble magatama and no one can give me a solid reliable answer...

anyone here know a good place to find them?

Luck Daruma outside of the Izumo rest area.

This thread probably belongs in the general section, but regardless what SupremeTentacle says is correct but the black glass marble can also be found dropped from any mob in kikaigahara through izumo open fields. (not dungeons)