Dratini has been around in Pokemon since the beginning. While most of the older pokemon aren’t in Pokemon Black or White, Dratini is still around.
Dratini evolves into Dragonair at level 30, which evolves into Dragonite at level 50.
Dratini is found by fishing around Dragonspiral Tower. You can fish it out of regular water, or swirling water, and they both have about the same chance of encounter.