Ok, I need help findinfg a bagon. The only reason it's in my dex is because..

To catch Bagon, you need Surf and Waterfall.

From Fallabor, go west into Rte. 114 and enter Meteor Falls.
Go across the bridge, go up the Waterfall and into the doorway. Then, go down the nearby ladder.
Now, go left all the way and go up, then up the ladder. There'll be an Old Couple there.
Go up the stairs and defeat the Dragon Tamer with the 2 Altarias. Jump down the ledges (LEFT SIDE!!).
Do NOT go all the way down! Before the last ledge, there'll be a hole going down. Take it.
Finally, Surf north and enter the room. This room contains Golbat, Solarock, TM02 (Dragon Claw), and BAGON.

How did you even know of Bagon from Dragon Tamer Nicolas?? He only has Altaria, never had a Bagon (I believe).
Anyway, He is found in Meteor Falls. I believe you know how to get there. So go up the Waterfall and enter the cave. Go down the stairs, unless you wanna go back. And then you can either go north or south from there. Going south wouldn't matter because it's a dead end. So go north and if you battled the Dragon Tamer then you obviously battled the Old Couple. So pass the trainers and start going down the ledges, STAYING AS FAR LEFT AS POSSIBLE. If you stayed as far LEFT as possible, you should come up to some stairs that lead you downstairs. Go down the stairs and go up to the water and use surf. Follow the path until you come up to the first appearance of land you see. Go onto the land and enter the cave. Once again, use surf and go to the end of that room. If you haven't been there before, you should see an item (TM02 Dragon Claw). Pick it up and run around that small area. You should eventually run into a Bagon, he isn't that rare. He's actually pretty common. You should also have no trouble catching him. He's between the levels 25 and 35.
Also, you don't need to trade Shelgon because just training him one level (if you found him at level 30 or 35) will evolve him into Shelgon. If you caught Bagon at level 25, you can either go with the trade, or you can train him 5 levels.

You will find it in meteor falls in the bottom floor in a small room

If youve got a shelgon and a salamence just breed either of them with a ditto or something from their egg group and the end will end up being a bagon.

The daycare to do this is just outside mauville city. Bagons are found in the same room in meteor falls as the tm dragon claw. Do a little work to find it, it's not all that hard to find.

Bagon is located at Mettor Falls.