So, you're looking for a guard-type sprocket then? You might take a look at the Odyssey MDS, it's got a protective guard on one side, and you can buy another guard to screw on the other side for extra protection if you need it. The biggest is a 30t, and it's made out of 7075 aluminium.

http://www.danscomp.com/458044.php?cat=PARTS