You get it after you Beat the Team Rocket boss in the radio tower in Goldenrod City. I believe this is after getting the 7th gym badge.
you go to pewter city and find the old man on a hill and he will give you the silver wing
eat it
In SoulSilver Version, the Silver Wing lies in the hands of the Director in Goldenrod City.
get silver or rainbow wing in radio tower
on your way to mount silver there is a house that you go in and they give steel wing.
If you're using heartgold, after beating the Pokemon league go to pewter city and an old man on a hill will give you the rainbow wing in soulsilver and the silver wing in heartgold.
get all 8 gym badges and profeser elm will give you a masterball and you can find lugia in the whirl islands and you need a silver wing
get the silver wing
You must defeat Archer of Team Rocket in the Radio Tower to get the Silver Wing.
You can find it on Whirl Islands, if you have a Silver Wind. If you play SoulSilver you also need Tidal Bell with the Silver Wing.
If your talking about Lugia from pokemon silver or SoulSilver then you need the silver wing for SIlver version. But for SoulSilver its a little different you will have to beat The kimono sisters then they you will have to meet them in the whirlpool islands watch a cutscene where they dance and lugia appear and fight Lugia where you can kill or catch it. But for HeartGold I dont remember how you get the wing for it but you must beat the game first before you catch Lugia.
from a blue old guy in pewter city