You have to beat the elite 4 and get the National Dex.
On Seafoam Islands.
Seafoam Islands
On Cinnabar Islands.
There are 7 non-event islands.
I don't really know about Pokemon leafgreen, but in Pokemon red you can get it in seafoam islands. Hope this helps!
1 islands
There are 2
seafoam islands
In the Seafoam Islands
i think there are only 3 islands Acually there are 7 islands
No, but you can catch some Johto Pokemon on the Sevii Islands.
Yes, articuno is at the end of seafoam islands. Try the boulders ;-)