Show Bill's grandpa a Pichu, and he'll give it to you.
you need a pichu and than give it to bill father by mistys gym and you will get a thunder stone.
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.
use a thunder stone on eevee
Moonstone everstone sun stone water stone fire stone leaf stone and thunder stone
the Pokemon that evolve with the help of a thunder stone are : pikachu +thunder stone = raichu, or eevee + thunder stone = jolteon.
At the Pokethlon Dome or at the Bug Catching Contest
You get it from Billy's Grandfather on route 25 after showing him some pokemon.
no one gives thunder stone you can find one underground well you have to search it
thunder Pokemon if it has evolutions
You can find the thunder stone in New Mauville
you can find a thunder stone by checking the UNDERGROUND
Pickachu and eeve can evolve with the thunder stone