It is only obtainable via a nintedo event, which will be held soon.
in prof .oaks
Get into pallet town
in the kanto region ,on the island of cinnabar
Pellet Town in Kanto below Viridian City
you use a action replay and use it to get oaks letter
get the oaks letter cheat on the action replay and talk to the guy in the pokemart
you go to route 224 and go to where the stone that is shiny, you face it and click a then prof. Oak will arrive and the island that leads to shaymin will show up. hope this helps =]
You get oaks letter from a special event that is on the internet or wifi. you get it from any poke-mart in sinnoh
u can put a cheat on ur action replay for oaks letter
you have to listen to prof. oaks radio show and he will say there are a bunch of murkrow at a certain route
yo go out of the veridian forest and talk to prof oaks aid and he gives it to you nooooooooooooooooooooooooooooooooooooooooo