I don't think that you can...seeings how its a Jhoto region Pokemon.
Do remember, once you beat Pokemon Colosseum/Pokemon XD Gale of Darkness you can trade Pokemon over to Ruby/Sapphire/Emerald if you have a purified Larvitar.
But I don't think you can actually catch it IN GAME though.
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Not naturally, no.
I am guessing this means "Where is the Pokemon Larvitar?". You can find it in the tall grass on Route 207 using Pokemon radar. If you mean "What does Larvitar evolve into?", then it first evolves into Pupitar, then Tyranitar.
To get a Tyranitar, you have to catch a Larvitar on 7 island and make it evolve.
find a caterpie in eterna forest then level it to level 7 and it will evolve into larvitar
Mt Silver.