Talk to the worker guy on the top floor of the Route 9 Shopping Mall. He will give you a Thunderstone.
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In Pokemon Black and White you can find a Thunderstone in these locations: Route 9 and Chargestone Cave. Sometimes, a dust cloud will appear and you could get evolution stones or Gems. These are the locations where you can possibly find a Thunderstone in: Wellspring Cave, Chargestone Cave, Mistralton Cave, Twist Mountain, Challenger's Cave, Victory Road, and Giant Chasm.
Eelectrik into Eelectross
Pikachu+Thunderstone=Raichu Eevee+Thunderstone=Jolteon These are the only 2 pokemon that evolve with the thunderstone in pokemon emerald
you can find one at the power plant
Tynamo will evolve into Elektrik will evolve into Elektross with a thunderstone in Pokemon White.