take a look in a petco or petland, they make all different sizes.
alternatively you could get some kind of floss pad and wrap them around the PH with zip ties or rubber bands. they will get filled up with debris pretty quick, you'll have to wash them or change them out at least once a week.
Well Jhale hit on it, the thinking was since it's water cooled the slow down in the incoming water may cause it damage. But since jhale says it won't then there really isn't any reservations except for the reduction of flow.