Where can you find a rampardos in Pokemon Diamond?

You can't find a Rampardos in Pokémon Diamond. Rampardos is a Pokémon that evolves from Cranidos, which is only available in Pokémon Pearl.

However, if you're playing Pokémon Diamond, you can still obtain a Rampardos through trading with someone who has a Cranidos from Pokémon Pearl.