Okay, it took me a long time to find a Bagon. You know the Meteor Falls Cave place, right? Well, just use Surf in some places and Waterfall in others, and eventually you will come into a thin, small cave with a thin stretch of water ahead of you. Now go Surf to the end of the cavern, where there is a small little piece of land. Basically, just run around there for a while (you're probably going to run into quite a few Zubats) and eventually you'll run into a Bagon. Stack up on Pokeballs, it will take a while to find one again (lots of more running and pointless Zubats).
Tips for raising Bagon: Now, like most Dragon-types, they take tons of Exp points to level up, and they evolve at high levels. Exp Share is good, just run around in Victory Road for a while battling some Golbats, a couple of Gravelers, maybe some Lairons, and just a pinch of Medichams, you will evolve him into Shelgon. Then it can handle itself (no more Exp Share). Here is a little chart:
Bagon >(level 38)> Shelgon
Shelgon >(level 45)> Salamence
Good luck catching that elusive Bagon!
Hope I helped!