I don't know where all of them are yet but I do know a few places you can get them, where, and how.

Kishiden- Hell Difficulty (You have to have a party of 5 members, and it takes multiple runs. I know you can get the Banana Magatama here for sure)

Room of Rites- Hell Difficulty (You can solo this one, takes multiple runs)

Byakkou Cavern- Hell Difficulty (I don't remember if you have to have a full party or solo try both, multiple runs)

Honomusubi- Hard/Hell Difficulty (I'll check more into this one not sure if its solo or full party, multiple runs)

This is all I have for now, but I'll find out more dungeons and which maga's you can find there. Once I do I'll come and edit this!

-Kataumi