The Attleborough shields are good value, but Bushwear offer a bigger choice. Make sure you get the right size for the cut you decide on - too big looks silly, and too small is embarassing!
You can do the cut by eye using a wood saw after skinning out the head and before boiling. If you mess things up and the cut starts going off at the wrong angle, just turn the skull over and start again from the other side. Once the bone is dry any unevenness can be sanded down.
Alternately you can buy cutting jigs that are adjustable (ie long nose, short nose cut etc) and guarantee a straight cut but they cost, especially for the larger one size sold for red. Probably only worthwhile if you are doing a lot of heads - but why not club together with some mates and share one?