Islands 1-7 can be unlocked after completing the game and getting the national dex you can use the vermillion city harbor to board a ship to islands 1-3 on island 1 you must find the ruby inside mt ember when you do return to island 1's Pokemon center and give the ruby to the man near the machine he will give you a rainbow pass now you can go to islands 4-7.
1 islands
seafoam islands
I don't really know about Pokemon leafgreen, but in Pokemon red you can get it in seafoam islands. Hope this helps!
Artciuno is in seafoam islands.
You find the ruby and give it to celio
Level 50 Pokemon can be found on islands 5-7.
On Seafoam Islands.
Articuno is somewhere in seafoam islands.you can find it in the seafoam islands from fuchisia NOT the island with POKEMON mansion
On Cinnabar Islands.
Seafoam Islands
There are 7 non-event islands.
beat the game and get to the indigo or orange or whatever islands, where u find hoenn pokemon.