Trade it from Sapphire, Ruby or Emerald.
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No you do not need to catch Kyogre. You can get a Hoenn starter from a guy named Steven in Kanto.
Getting the Hoenn Pokemon is completely optional. Therefore, there is no chronological event that is placed after getting your choice of Hoenn starter. Just try doing some other extra events.
I think you have to trade to see them. Actually, you have to trade to see lots of things in Hoenn. But it'd be worth it; you get a Johto starter for completing the Hoenn Dex.
Torchic, Mudkip and Treecko.
You have to complete the Hoenn pokedex to get one from Prof. Birch. That includes catching all the Pokemon in there. This is impossible, but you can get a Johto starter if you finish your Hoenn pokedex and show it to professor Birch.