...after leaving the Vermilion City Pokémon Fan Club house with Copycat's doll, Latios [or Latias] will roam Kanto.
Source: Bulbapedia/ Roaming Pokemon
In SoulSilver, you get Latios. If you want Latias, you trade from HeartGold. There was an Mystery Gift event for an Enigma Stone, but has long since passed.
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