Where do you get Kyogre?

Answer 1

In Pokemon Sapphire, you can capture Kyogre after encountering it once in the cave where you met Archie. Kyogre will flee to Sootopolis City and arrive in the Cave of Origin, north of Sootopolis City.

You will need to trade for Kyogre from Pokemon Sapphire or Emerald to obtain it in Pokemon Ruby.

In Pokemon Emerald, Kyogre can be found in Marine Cave when the guy at the Weather Institute announce bad weather in the sea.