If you visit Muramasa's store, you can score some free yellow essence from him simply by attacking him. Try attacking him with a weapon like a nunchuck, and he'll begin dropping his yellow essence. But don't push it! If you hit him too many times, he'll fall due to his injuries and you will be hit with a Game Over. Save before you try this, so you can see how many times you can hit him before he dies and get the maximum amount of yellow essence from him.