I can see a few ways for The Walking Dead comic and TV show to end, but someone finding a cure is not one of them. Mainly because no one has any resources to be able to discover a cure, but also because The Walking Dead’s zombies are of the massively decomposing variety, and any “cure” someone found would merely let them die. If a cure doesn’t actually cure people, then there’s no point in using that as opposed to something else.
What will that something else be? Well, I imagine when Kirkman wants to end the comic or the show, the survivors will simply win the war of attrition with the zombies. Maybe they’ll kill enough zombies to make the omnipresent threat natural; maybe enough time will pass that mother nature will help, and many zombies will decompose to the point of being unable to attack anything. I’m getting the sense that the survivors will find a new place to stay, the zombie attacks will get fewer and farther apart, and they’ll be able to continue building a city and rebuilding civilization. This will all be with a final montage, of course — it’s not like we’re going to get to watch things get slowly better for either the comics or the TV show cast.
For the record, I think Rick’s chances of surviving to see this semi-happy ending are about 1 in 8,262,047. He’ll sacrifice himself to save Carl and the group, and Carl will have to kill his zombie dad, at which point he’ll lead the group as a young man to the end I described above. This may happen earlier in the TV show in Andrew Lincoln decides to leave it before the end — which would be stupid, given that The Walking Dead is one of the most popular shows on TV and I don’t see much of a movie career in his future. But since AMC is going to keep this show going for as long as they can, it’s possible Lincoln could get bored by year 8 or so and want out.
At any rate, this will lead to a sequel series where Carl and Lizzy (the new crazy girl from season 4, who will bond with Carl over their pasts as young psychopaths) have a dozen babies, which the elderly Daryl trains to become an elite zombie-fighting team. These 12 young men and women roam the land, trying to eliminate all remaining zombies for the safety as all. They, as well as their show, will be known as “Carl’s Juniors.”