You go to the Dragon's Den in Blackthorn City. And before you go to the shrine, there is water that you can surf on. That water has dratini in them. Catch the dratini and train it to level 30. There you have a dragonair. Then, to level 55 and it becomes a dragonite.
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OR you can go get super rod. But I think you get it in Kanto, so...
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Level 55
Level 55 into Dragonite.
It evolves at level 30 in dragonair and again at level 55 in to dragonite
Give it an experience Share and let another Pokemon battle for it.
Dratini can be found in Dragon's Den, level it up and evolve it.