you can't that was only in gold and silver
You don't get either in firered, to get lugia and ho-oh get them from Pokemon colosseum and Pokemon xd gale of darkness.
you can only get it by trading it from emerald GBA you need a GBA cable a defeated gale of darkness +emerald
cheats
You get the Rainbow Wing from a man in Pewter City next to the poke mart
go to pewter city and you will see a old man on a ledge.he will give you the rainbow wing.
You get Silver wing After defeating the team rocket in the radio tower POKEMON SILVER ONLY. Rainbow wing Is found much later in the game After the Elite four In pewter City POKEMON SILVER ONLY.
After you beat the last team rocket executive on the observation deck, in the radio tower in goldenrod city, you get the rainbow wing from the Rainbow wing from the director.
in goldenrod city
in pewter city
Once you get to pewter city a man somewhere in the city outside will give you the rainbow wing (how easy).
Those items aren't in LeafGreen.
You son of a... Dumb a#$