Okay, some good Dragon types are Flygon, Haxorus, Druddigon, and HYDREIGON!
For Flygon, you can catch a Trapinch in the Desert Resort in the desert part or, if you have progressed quite a lot in the game, you can even catch a Vibrava in the exterior part of Reversal Mountain.
For Haxorus, you can easily catch an Axew in Mistralton Cave and train it up. You will have quite some time to train it up before facing the Elite Four.
For Druddigon, you will have to make your way all the way to Victory Road to catch it.
As for Hydreigon, you can catch a Zweilous in Victory Road as well, but you will have to train it to level 64 to evolve it, which means you already would have beaten the Elite Four way back the first time.
So all in all, even though Hydreigon is the best of the lot, you can't really catch it early and will have to wait till post-game (Or, you have to be immensely patient to get it earlier), and by that time, other cool dragons will already begin to appear on the scene.
Haxorus is the next best if you have the patience to train it up, (and Flygon too) and it will be a really awesome addition to your team.
Hope I helped :)