Well in my time playing the game versions sadly you can only get 1 crystal per boss for the Lich, Bandit boss, Foreman dragon, Mad chimera, and Bunshido you also can get one free from Plush's area other than that only thing I can tell you is go dungeon delving and hope you find one. I've had one of the kobolds in the crystal cave dungeon in the construction zone drop one but it's likely rare.