You need surf, you'll find on at New Mauville (the route is under the cycle bridge to the east) There'll be a thunder stone in the complex. You can also collect Yellow Shards from using Dive in the Ocean and trade them for a Thunderstone. Otherwise you can simply trade with FireRed/LeafGreen with a Pokemon holding a Thunderstone.
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Pikachu+Thunderstone=Raichu Eevee+Thunderstone=Jolteon These are the only 2 pokemon that evolve with the thunderstone in pokemon emerald
Tynamo will evolve into Elektrik will evolve into Elektross with a thunderstone in Pokemon White.
The Pokemon that evolve using a thunderstone are: Pikachu into Raichu Eevee to get a Jolteon
Thunderstone can be used on Pikachu and Eevee, it evolves Pikachu into a Raichu and Eevee into a Jolteon.
You get it by digging underground.