you get it in the house on silence bridge
in the somewhat tall grass. or when you are fishing with a fishing pole. or when you run into a Pokemon when surfing on water.
go fishing
Around kanto certain fisherman give you fishing rods.
at jubilife or enterna city
magikarp
yes you do need a fishing pole to catch mitotic in Pokemon black but you cant catch that Pokemon you have to catch the first evolvation and then you train it until in evolve into a mitotic
You can mostly get Water type Pokemon by fishing or surfing on water. Like the ocean or sea.
Heck if I know!! Haha
go to the lake of rage
go fishing in route 12.use superod
Fishing pole? You use a Super Rod to catch it. There is no fishing poles Pokemon,only Old Rods,Good rods, and of course Super Rods.
Once you have a fishing pole in Pokemon go to a large body of water and cast the rod, after a little bit you will get a bite and a Pokemon battle will begin.