Unsullied Heart Farming
They drop from the stronger mobs in villages and towns. Some enemies have the small hp bars, Unsullied Hearts drop from the ones with the big HP bars and are usually harder to kill. Dive into towns and murder everyone you can find and you’ll have plenty in no time.
Note: I believe they even hotfixed the drop rate for the hearts a few days ago.
Also, they are nearly a guaranteed drop from V blood carriers.
If you’re high enough level, you can bypass farming the hearts and craft the greater blood essences yourself.