first you have to beat the elite four. After you capture 60 POKEMON talk to prof oak he will give you the national dex. You will give him your island pass and he will turn it into a rainbow pass allowing you to go to all the the islands.
Seven Island
Island Seven. In the ruins.
Larvitar is in Seavault Canyon, which is on Seven Island. (Need rainbow pass to get to Seven Island).
It is on Seven Island, to the north.
On Seven Island in Sevault Canyon.
northern seven island
use the mystery gift.
Cinnibar island if your going in order.
Cubones can be caught on Seven Island.
Larvitar can be found in the grass in Sevault Canyon on Seven Island.
unown appear in the ruins at bottom of 7 island
Onix can be found on seven island, six island and rock tunnel.