Where do you find a dome fossil in soul silver?

You can't find a Dome Fossil in Pokémon SoulSilver. Dome Fossils are only found in Pokémon Diamond and Pearl, and their evolution, Kabutops, is only available through trading in SoulSilver.

If you're looking to get a Kabutops in SoulSilver, you'll need to find someone willing to trade you one, or find someone with a Dome Fossil who can trade it to you.