Currently VERY difficult to get these; invasions almost never happen, making Oathbound protection almost impossible to achieve. Ska need to provide a way to farm them from enemies.

You can get 3 of these in bol gerahn after getting the luminstone in corvius mire, they are where the nameless aeromancer hunt is

From the starting Obelisk, head east past the Talking Head's kiosk and grapple up to the path leading to the Praying Giants. Along the way, look for the following a pillar with a Stone Circle just before the next two grappling points. Grapple up to the pillar and activate the Stone Circle to reveal several luminous platforms heading west.

These can now be obtained in fated mage hunts, just got the two I needed from completing them, make sure you check your rewards on each mage and do the one that gives these.