You will need to trade for it or transfer it from the older games to the Pokemon Bank.
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Dialga is not in that Pokemon Emerald. Back when that game was made, Dialga didn't exist.
you do not find dialga in Pokemon pearl
we find dialga in map 3 in Pokemon deluge...
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.
You can't get dialga in firered.