Were do you get a light ball in Pokemon Diamond?

You get a light ball in Pokemon Emerald, and then transfer it to Pokemon Diamond, Pearl, or Platinum via the migration method you obtain after beating the Elite Four and obtaining the National Pokedex. You can also get one from a Pikachu in the Pokemon Mansion. If you have an Action Replay, here is the code for it:

Light Ball x999

94000130 fcff0000

b21c4d28 00000000

b0000004 00000000

00000890 03e700ec

d2000000 00000000

It should work, I'm so sorry if it doesn't.