I think that 540 motors are used in most 1/2A starters; at least in the Miller, Sullivan and Astro Flight starters that I've seen anyways.

Here's one at [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGGA8&P=7]Tower[/link]

Of course you'll want something that can handle 12V. This one at [link=http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=DCM-216&type=store]allelectronics.com[/link] should do the trick. It looks like they have a little motor similar to the one I used, [link=http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=DCM-141&type=store]click here[/link].