Where do you get the rainbow wing on Pokemon gold?

I'm completely sure, I only know Heartgold but they have such a similar way to get things, After getting your seventh badge, go to Goldenrod City's radio station, team rocket will be there, beat ALL of team rocket, (INCLUDING THE EXECUTIVE!) someone will give you the rainbow wing.

In Silver and Soulsilver: go to Pewter City and talk to a man. The man will give you a Rainbow wing. In Silver Ho-oh will be Lv 70, so take Pokemon near its level.; In Soulsilver HO-OH is level 75! BE CAREFUL! My advice is to catch LUGIA with a great ball and catch HO-OH with your Master Ball. (unless you have cheats, then you can catch all Pokemon with a pokeball.)
You get the Rainbow Wing from an npc called Old Man in Pewter City.

In Pokemon HeartGold the rainbow wing can be obtained when you defeat Team Rocket at Goldenrod City, an event following on from the previous defeat of Team Rocket at Mahogany Town.