You can purchase the elemental stones from level 4 in the Celadon Department store. Prices vary, but all of them sit around the 2,000 PokeDollar mark.
Or you can find them at the powerplant.
If im not mistaken, you can buy all stones except moon stones at the celadon mart, the giant mart with multiple floors. check it out, i may be a little rusty.
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Evolve the Eevee you get given at the top (using the back entrance) of the Pokemon 'Mansion' in Celadon using a Thunderstone.
The Pokémon that evolve via the Thunderstone are Pikachu which evolves into Raichu and Eevee which evolves into Jolteon.
Pikachu+Thunderstone=Raichu Eevee+Thunderstone=Jolteon These are the only 2 pokemon that evolve with the thunderstone in pokemon emerald
You can actually purchase thunderstones on the 4th floor of the Celadon City Dept Store you can buy as many as you want.
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.