Drops from Provenance Watcher it seems. No indication of anything else it drops from and no sign of Arise yet. Unlike other voidwatch it drops from the NM itself, don't know if it can also drop from the personal chest.

Provenance Watcher itself doesn't look like a particularly difficult fight being pretty much a 4-5 minute zerg, but the pre-requisites to fight it (All Jeuno clears just to access the zone and 3/3 petrifact KIs) means supply will be limited for the forseeable future. Expect 20 mil+ unless you're lucky enough to get one from the fight itself.