You must beat the whole game (beat the elite four and champion). Then head to Snowpoint City and talk to Cynthia. You should then be taken on a ship to the Fight Area. Go straight up from there and you'll be at route 225.
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Rt 211
Route 225 is not a route that can be found in Pokemon Firered, route 225 is in Pokemon Diamond, Pearl, and Platinum. To get there is Diamond Pearl and Platinum, you have to see every Pokemon in Sinnoh, then talk to Prof. Rowan and get the national dex.
In Routes 225 and 226 using the Pokeradar.
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You can catch one at Route 225 or at Route 226 (using the PokeRadar).