How do you get the red orb in soul silver?

You can't actually get a Red Orb in Pokémon SoulSilver. The Red Orb is an item from Pokémon Ruby and Sapphire, and it's used to awaken Groudon.

SoulSilver features a different legendary trio: Raikou, Entei, and Suicune. You can obtain them after defeating the Elite Four by searching for them in specific locations across Johto.

Perhaps you're thinking of the Red Scale in SoulSilver. This item can be found in the Hidden Grotto on Route 47, and it is needed to evolve Seadra into Kingdra.

If you're looking for a specific item or Pokémon, please clarify what you're searching for, and I'll be happy to help!