The Light Ball was introduced in Pokemon Black and White. It was not available in previous generations, including Diamond and Pearl.
However, there are some ways to get a Light Ball in Diamond and Pearl using glitches or external tools, but these methods are often considered cheating and are not recommended for legitimate gameplay.
If you are looking for a specific item in Pokemon Diamond or Pearl, it's best to consult a reliable guide or Pokedex that is specific to those games.