One way is to catch a Pupitar on Route 15 and evolve it. Another way is to catch Tyranitar outright. Tyranitar has a 5% chance of being one of the Pokemon you encounter in shaking grass on Route 15.
You can also catch and evolve a Lavitar from the Rugged Mountain Dream World Area. And of course, you could always trade/migrate for any of the Pokemon in Tyranitar's evolution line in order to get it.