How do you get the light ball in diamond?

You can't actually get the Light Ball in Pokémon Diamond.

The Light Ball is an item introduced in Pokémon Black and White, and it boosts the power of Normal-type moves. It doesn't exist in Pokémon Diamond.

If you're looking for an item to boost Normal-type moves in Diamond, you might want to try the Choice Band, which boosts the power of a Pokémon's chosen move by 50%, but prevents it from switching out.