Where do you get a swampert on soul silver?

You can get a Swampert in Pokémon SoulSilver by evolving a Mudkip.

Here's how:

1. Catch a Mudkip: You can find Mudkip in the Route 120. It is a water-type Pokémon.

2. Level it up: Level up your Mudkip to level 16. This will evolve it into a Marshtomp.

3. Level it up further: Continue leveling up your Marshtomp to level 36. This will evolve it into a Swampert.

Note: You can also trade for a Swampert, but evolving your Mudkip is the most common way to get one in Pokémon SoulSilver.