You have to beat the game first. Then talk to Dawn's sister near the south of Sandgem Town. She will tell you of one swarm on one route
You can get Larvitar on Route 206 Swarms. Use a Poke Radar. Use Repeat balls after you catch the first one
BTW, if Dawn's sister doesn't say the pokemon swarm you want, turn the timer 24 hours up (a whole day) and she will tell of a different swarm.
hope this helps