It's just a large size lip seal. Make sure you get one with a stainless spring or simply install it upside down and grease the spring. Inner diameter you can measure off the rudder post, outer seal diameter off the tube.
As Happy pointed out, the Neoprene gaiter is simpler to install as it doesn't require hauling out or dropping the rudder. If you're hauled out anyways, it's no big deal - you don't have to get it all the way out of the hull, just lower it enough to get the new seal over the top. Requires taking the quadrant and everything else off though.