You can find a Dragonite in Pokémon Black and White at the Dragonspiral Tower in the Rippling Water although there is only a 1% chance you will find one.
Therefore it is better if you catch a Dratini or Dragonair which can be found at the same location in the Rippling Water and when Fishing and Level it up to Level 55 to evolve it into a Dragonite.
Dratini has a 65% chance to appear when Fishing and a 60% chance to appear in the Rippling Water. Whilst Dragonair has a 5% chance to appear when Fishing and a 9% chance to appear in Rippling Water.
Hope I helped :)