You can't. It's exclusive to pokemon black (or you can poketransfer or trade)
In pokemon black, it's a swarm pokemon on route 11. You'll see swarm notifications on bulletin boards in gates
He can't be found nor can his previous from, Shroomish, but if you can transfer him from a Generation IV game
Shroomish
In the Pokemon video games, Shroomish is able to learn Flash via the HM05. Flash is the only HM move which can be taught to Shroomish.
The Pokemon exclusive to the Black version are: Weedle Kakuna Beedrill Murkrow Houndour Houndoom Shroomish Breloom Plusle Volbeat Hounchrow Cottonee Whimsicott Gothita Gothorita Gothitelle Vullaby Mandibuzz Tornadus Reshiram
Shroomish due to it being a Grass-type is weak against Ice, Fire, Bug and Flying-type attacks.
It would be in a swarm. It would be on random days though. Shroomish is in Route 11 during a swarm, but unfortunately, Shroomish is only in Pokemon Black.
Shroomish is a Grass type pokemon.
Shroomish > lvl 23 > Breloom
Shroomish is a rare Pokemon in Ruby and Saphire but you can find it in the Petalburg woods.
Shroomish is not available to be caught in the wild in Pokemon FireRed, it must be traded from Ruby or Sapphire.
Shroomish > lvl 23 > Breloom
He can't be found nor can his previous from, Shroomish, but if you can transfer him from a Generation IV game
Shroomish evolves into Breloom, starting at level 23.
Breloom!
yes you can.
Shroomish
Shroomish is #285 in the national pokedex, and it is a Grass type Pokemon.