Dragonite has been around in Pokemon since the beginning. While most of the older pokemon aren’t in Pokemon Black or White, Dragonite is still around, and is still one of the more powerful normal pokemon you can get.
Dragonite evolves from a Dragonair at level 50, which evolves from a Dratini at level 30.
Dragonite is found by fishing around Dragonspiral Tower. You can only fish it out of swirling water, with about a 1% chance of encounter.
Alternatively, you can catch Dratini or Dragonair with much better odds of encounter, and level one of them up to get your Dragonite.