Where do you catch a Sudowoodo on Soul silver?

You can't catch a Sudowoodo in SoulSilver!

Sudowoodo is a Johto region Pokémon, and SoulSilver is a Johto region game. Since Sudowoodo is native to the Johto region, you won't find it in the wild.

You can, however, obtain Sudowoodo in SoulSilver through trading with someone who has one.