you can find one at the power plant
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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.
Pikachu+Thunderstone=Raichu Eevee+Thunderstone=Jolteon These are the only 2 pokemon that evolve with the thunderstone in pokemon emerald
Piplup does not exist in Pokemon LeafGreen or FireRed.
Talk to the worker guy on the top floor of the Route 9 Shopping Mall. He will give you a Thunderstone.
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.