You can get green essence by beating Green Slimes in the Swamp. Keep looking around there until you find the Green Slime Creatures then kill it to get their essence. For the Diabolic Herb, they're found in the Dark Swamp, I can't remember how to unlock that place but perhaps progressing through the Lizard quests might do the job. For the Oak Woods, they're pretty much available in the forest, just keep looking around and a prompt will appear asking if your want to chop oak wood or not.