Where do you get traps for base Pokemon diamond?

You can't get traps in Pokémon Diamond. Traps are not a mechanic in the game.

It sounds like you might be thinking of trapping Pokémon, which is a strategy used in competitive battling. This involves using a Pokémon with a move that prevents the opponent from switching out, such as Whirlwind or Roar.

If you're looking to catch specific Pokémon in Pokémon Diamond, you'll need to use Poké Balls and find them in the wild.