A Thunder Stone is an evolutionary stone that allows certain Pokemon to evolve when exposed to it. In the third generation of Pokemon games, there is one Thunder Stone in New Mauville, and they can be had from the Diving Treasure Hunter for Yellow Shards.
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Moonstone everstone sun stone water stone fire stone leaf stone and thunder stone
At the Pokethlon Dome or at the Bug Catching Contest
You get it from Billy's Grandfather on route 25 after showing him some pokemon.
After you battle and beat the gym
the Pokemon that evolve with the help of a thunder stone are : pikachu +thunder stone = raichu, or eevee + thunder stone = jolteon.