Black 2 and White 2. After completing the pokedex, you get a pass to go to the Nature Preserve. Go to Mistralton City and fly on the plane to get to the Preserve. If you fish, you can get a Magikarp from levels 1-100. If you fish on ripples, You can find Magikarp levels 1-100, but you can find Gyarados levels 1-100, too. Gyarados is rare, though. Hope this helps.