What is the best place to find essences? I have up to farina's shrine unlocked, and have gotten a qitari essence by bowling through sylestis in Troll's Bright caverns, with level 20 pets. I am hoping for a enhanced vulnyx essence, but i'd be happy with whatever!

You can get enhanced essences in the diamond shop, or in the player trades for gold. Other than that you can get essences when you nurture eggs and hatchlings, it's a random drop and most often just regular essences, or from battling, usually you get the essence of the sylestie you just fought. You can also get a essence drop from Jinxie's Jamboree but it's rare.

Just going to add on to Fernwhisper's comment that essences also have a rare chance of being obtained from mission satchels.

If you're looking for a particular species essence, you should do nurturing. Use the advanced search to find all the eggs/hatchling of that species and nurture. Essences obtained from nurturing and battling will be of the species you were nurturing/battling at the time you got the essence. Whereas the other methods are random species drop.