Get 16 badges, get rock climb from Prof. Oak, go to Cinnabar Isl, head to the rocky edge that's facing the back of the Pokemon center and rock climb it, head right...then you see a Poke-ball items thing I think that's it if I remember :/
Kanto: Cinnabar Island Pokéwalker: Amity Meadow (2500+ Steps)
Source
Cinnabar Island
Source: Bulbapedia/ Cinnabar Island