Were Do You Catch A Bagon In Pokemon Platinum?

First, go to celestic town. Then, go to the first patch of grass to the right of town. Then, use the pokeradar. It's easier when you leave the fog. I don't know why but it took me forever when I used defog but the one time I left it, I saw  bagon in three tries. Be slightly patient though, it will take a little but not too long.  (I HOPE YOU GET 'EM!)

The pokedex does not say all areas where certain Pokemon are found. Bagon can be found in Wayward Cave (very deep in it, though), northern Route 210, and very, very (trust me it is almost impossible, go for choice 1 or 2, I suggest 2) rarely in the Great Marsh.
Easiest way (if you have GBA games): Migrate it.
I've have tried and found Bagon in ALL these places. None of these are rumors or lies.

--source-- My game.

Near solaceon town on the left side you have to use the poke rader if you do not have a poke rader I can give you  1 2 3 4 5 6 7 8 9 10 if you give me your pal number I well give you 11 ok?

You go to the place near celestic town and solaceon town and then use your pokeradar at the grass to catch the Bagon. You get a pokeradar when you get the national pokedex and talk to Proffesor Rowan. When you talk to him, you will get a pkeradar. Hope I helped you.

P.S
(Happy Hunting for Bagon!)

You can catch bagon at celestic town. Remember to use your pokeradar

You go on rout 210 and you know where the fog is use a poke radar there P.S Happy Hunting

Route 210 (north) pokeradar

Nere celestic town use pokeradar

You catch it in pokemon red version