"Gift from the director after saving Goldenrod Radio Tower"
-http://bulbapedia.bulbagarden.net/wiki/Rainbow_Wing#Rainbow_Wing
Chat with our AI personalities
After you beat Archer (The neo team Rocket Leader) the directer gives it to you
To the right of the town, there is a small hill will an old man standing on it. Talk to him and he will give to the Rainbow wing for Soulsilver, or the Silver wing for Heartgold.
the rainbow wing is your key into the bell tower. you just walk in and as you talk to the monk blocking your way, it will hover above your head and he will let you through. go through the tower until you get to the roof. the kimono girls will summon ho-oh with the help of your clear bell (if your playing soulsilver and beat the elite 4, you don't need a clear bell) in heartgold ho-oh is level 45, in soulsilver he's level 70, but you can only catch him once you've reached kanto and gotten the rainbow wing. (same for catching lugia in heartgold)
You get the Rainbow Wing from a man in Pewter City next to the poke mart
you need to have ho-oh