I think you can find one in sunyshore.
its underground
You get it by digging underground.
The Underground or Sunyshore City
one place is in the underground
You can get one in sunnyshore city or underGround
go underground and get a thunderstone and use it on pikachu
In Pokemon diamond (what I have) you can get it underground or I think you can get it at the huge pokemarket in one of the cities.
i believe you go underground and dig it up.
You can get one in sunyshore or u can dig for one underground
The most easy way to get one is to dug it up in the underground
The thunderstone can be found at Sunyshore City. Go to the Vista lighthouse and go down the stairs. At the bottom there will be a thunderstone. You can also dig these up in the underground.
I'm not exactly sure ,but i think you have to go underground and dug it up.