Route 119:
* There's a chance to find one in the tall grass.
Route 123:
* You can find one in the tall grass, but the chance is still pretty low.
Granite Cave (Post-game):
* You have a good chance to find one here after you beat the game.
Other Methods:
* Trade: You can try to trade with other players for a Thunder Stone.
* Breeding: You can breed a Pokémon holding a Thunder Stone with another Pokémon to increase the chance of your offspring hatching with the ability to evolve through Thunder Stone.
Important Note: The Thunder Stone is used to evolve Pokémon such as Pikachu, Pichu, and Eevee into Raichu, Pikachu, and Jolteon, respectively.