Dragonitis are in the dragons den. Catch one and evolve it. personally i dont think it is worth it after he is not traded and can't evolve into Dragonite.
at level 55, dragonair turns into dragoniteand you need to evolve dratini for dragonite to get dratini after u cleared the test with clares papa you go back to the dragon's den and he will give u level 15 dratini.
To get a Dragonair, you must catch a Dratini. They can be found in the waters of the Dragon's Den or by fishing in the ponds south of Blackthorn City. The easiest place to find one by my experience is in the Dragon's Den though. Dratini evolves into Dragonair at level 30 and then into Dragonite at level 55.
Level 55
magikarp dratini dragonair
No, you need to evolve a Dragonair.
You must evolve Dragonair, which you get in the Dragon's Den.
In Pokémon HeartGold, Dragonair can primarily be found by fishing in the Dragon's Den, located behind the Blackthorn City gym. Players can also encounter Dragonair in the Safari Zone’s Wetland area, where it appears at a low encounter rate. Additionally, you can evolve a Dratini, which can be caught in the same locations, into Dragonair at level 30.
Dragon's Den (Blackhorn City)
You level up a Dragonair to Level55 then - hopefully - it will evolve into a Dragonite.
Dragonair are found rarely at Mt. Coronet.
You must evolve Dragonair (available from the Dragon's Den; use super rod) to get it.
in map 4
Mount silver, or the elite four (With exp share.)
First you have to play the casino/games place in goldenrod city (where you battle whitney) and earn game points by playing voltorb. Once you get enough points you can redeem them for a dragonair. Hope it helps. P.S. the games place is not inside the gym.