3 boss mobs all located in the Penumbra Catacombs;
HumbadaThis guy Area Nukes, takes an MP along for NSD
Sursunabu
ErshkigalThis guy Area Nukes, takes an MP along for NSD

All stand an equal chance of dropping it.
Simply select the temple of your choice Redeemed or Unredeemed ( Faction Standing doesn't matter here ), and make your way to the spirit room, once inside you can zone through a door to your East or your West, whichever side you choose run down into what appears a dead end and jump into the black holes. Welcome to the Catacombs.

This can be done with care, and skill by a single crat' and MP.
Best method we've established is have a 'crat lead the way, first landing a Shadowlands Calm on the Catacombs mobs, then
re-calming them with a Rubi-Ka Calm. Simply calm your way to the boss mobs.
For Humbada and Ershkigal you WILL need an MP for Nano Shutdown, these guys area nuke for ALOT !!