Just be lucky. Keep fishing :P
fight water Pokemon ( by fishing or surfing)
well you can get 1 in route 218 from a fisher man
every time you see it say got a nibble press a and soon you will hook a Pokemon
the fisherman in right of mauville in the other side in the mosdeep or sootopolis (i'm not sure ehh)
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.
Fishing in Mt. Coronet
By fishing on Route 220.
magikarp
at jubilife or enterna city
you get it in the house on silence bridge
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
Mt Coronet by fishing in certain areas
canalave city
go to the lake of rage
Heck if I know!! Haha
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.