Spirit Essence at high levels?

I'm level 17 and I can't find ANY Spirit Essences. I didn't know I'd need them so earlier on, when I had more, I sold them. Now I only ever get Intense Spirit Essence, and that doesn't work for the Main Tower Upgrade. I've closed all the lower level Rifts, so I have no clue what to do.
Any locations I can loot them, vendors who sell them, quests I can do to get them?
 
Edit:
Okay, I got one so far.

Quest - Deploy Rhys and Evangeline
How to Unlock: complete Locate Rhys and Evangeline
Preferred Specialist and Rewards: Forces, Influence, and Spirit Essence

Found a solution. Still, lower level items should still be more readily available even at higher levels. It's stupid for them to suddenly turn into a rarity.