It isn't available separately. It is just a ball bearing fitted into the cylinder head when manufacturered. Look at the bottom of your cylinder head, the brass ring in the oil feed into the head is where it lives. As the oil is fed into the head from the block, the ball bearing is pushed up into a recess and the oil flows into the oil ways in the head for cam / lifters etc. When engine is switched off the oil pressure drops, and so the ball bearing falls back down, keeping some oil in the cylinder head oil ways. It is there purely to stop all of the oil draining out of the oilways when engine is off.