From FG general areas maybe, Lotus is bountiful if you run Tomb of the Heretic though as every Scion boss, Magi and Morg have a chance of dropping them. Morg can even drop Celestial Essence for the masochistic.
For anyone that doesn’t want to run Tomb or can’t afford the keys, Celestial Totems are also not bad on drop rates and easy to run if you rift to Burrwitch and check Galeslice’s area and the spot on the way to Abaddoth’s church. The Totem fights are far more difficult than other Totems though.