You can get it in Cerulean but pikachu won't let you use it on him in yellow.
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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.
the Pokemon that evolve with the help of a thunder stone are : pikachu +thunder stone = raichu, or eevee + thunder stone = jolteon.
you can find a thunder stone by checking the UNDERGROUND
You can't, it needs a Thunder Stone.
Pikachu and Eevee both need a Thunder Stone to evolve.