You can teach Thundershock to certain Electric-type Pokemon like Pikachu, Raichu, Magnemite, Magneton, and Electabuzz. You can find these Pokemon in various locations throughout the game, such as:
* Pikachu: Route 34, Ilex Forest, and Ecruteak City (rare)
* Raichu: Trade from a Pikachu that has learned Thunderstone
* Magnemite: Route 38, Power Plant, and Goldenrod City
* Magneton: Evolve Magnemite
* Electabuzz: Power Plant and Goldenrod City
Once you find an Electric-type Pokemon that can learn Thundershock, you can teach it the move by going to a move tutor. Move tutors are found in various locations throughout Johto.