By getting all of the 8 badges - making your way through victory road and then taking them on in Indigo Plateau.
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one of the elite 4 has one
you get it after you beat the elite 4 go to the desert and catch one.
nation pokedex
the most you can get is in jubilife, in the pokewatch building after you defeated the elite 4.
You can't catch Dialga in Pokemon White. After you beat the the Elite 4, you can use the PokeTransfer to receive a Dialga from Pokemon Diamond, or Platnium.