Where is the cape in soul silver?

The cape in Soul Silver is located in Route 45. Once you have acquired the National Pokédex from Professor Elm and Surf from Mr. Pokémon in Olivine City, go all the way to Route 45. After clearing the tall grassy area, use Surf near the small body of water to the left. Follow the shoreline until you find the cape, which will be a long, stretched out landmass.