You can't, you either have to trade one from FireRed or LeafGreen, or trade the purified Tyranitar from Pokemon Colusseum and breed it.
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You can catch Larvitar at the Rock PokeSpot on Pokemon XD for the GameCube,You can trade it onto Sapphire after you've beated XD001 (Shadow Lugia)
Larvitar can be found in Mt.Silver.
No you cannot catch Larvitar in Pokémon Emerald.
mt.silver
Not naturally, no.