complete the sinnoh pokedex and talk to professor rowan.
there is none
you can get bagon on Pokemon platinum on route 210 using poke radar
right after you get national dex
The Poke Radar is not available in Pokemon Sapphire. The Poke Radar is an item exclusive to Pokemon Diamond, Pearl, Platinum, X and Y.
use poke radar on route 210
Route 224 Poke Radar
You can find it at route 228 with the poke radar
Route 212 with the poke Radar
lake valor [use pok'e radar]
route 210 (use poke radar)
Route 210 South (Poke-Radar)
On route 229 with the poke radar(rare)