You tell me
i got 1 by using rock smash on a rock in a cave on route 47
In cerulean cave. But before you go there, you have to have all the Kanto badges and the HM08 Rock Climb
in soul silver
you go to the cerulean cave and then you use rock smash on huge boulders in kanto
At Mt. Silver cave. Next to the entrance as you walk in will be a body of water with a waterfall. go up there. Rest you can figure out yourself (At top, cave, rock climb then its there)
Cerulean Cave
Hes on top of the cave just west of the Pokemon league gotta have strenght rock smash and rock climb
In the Cerulean Cave, you can find it on BF1 in the north-east corner. You will need Rock Climb.
you can find it in cerulean cave in the northeast corner of b1fyou to have a pokemon that knows rock climb
go to icefall cave
mewtwo
it is in cerulean cave