Capture Pikachu
If you don't already have a Pikachu, you can catch one in the Pokemon Mansion on Route 212. The fastest way to reach Route 212 is by walking south from Hearthome City; it's also accessible by heading west from Pastoria City. Upon entering the Pokemon Mansion, walk north and go through the back door to reach the trophy garden. There is a 10 percent chance Pikachu appears. All Pikachus in the trophy garden are level 18.
Sunyshore City
The quickest way to get a Thunderstone in "Pokemon Diamond" is to travel to Sunyshore City. To get to Sunyshore City, go to the Valor Lakefront and walk east on Route 222. If this is your first visit to Sunyshore City, you meet Flint, who asks you to defeat the Elite Four. After Flint finishes talking to you, travel up the stairs and walk east until you can go no further. Go down the southern stairs to pick up the item ball. The Thunderstone is inside the ball.
Sinnoh Underground
If you already used or sold the Thunderstone in Sunyshore City, you can also find one in the Sinnoh underground. To enter the underground, get the Explore Kit from a man in Eterna City. When you use the Explore Kit, you go underground and enter a series of caves. Tap on the walls using the Nintendo DS stylus to start an excavation mini-game. Tap the panels on the wall to break them with your pick or hammer. If you uncover the correct panels, you find a random item, such as a Thunderstone. If you break too many rocks in the wall, it collapses, ending the mini-game. It is very rare to find a Thunderstone this way before you get the National Pokedex. After getting the National Pokedex, you have a slightly better chance of finding one.
Evolve Pikachu
To evolve Pikachu into Raichu, you must use the Thunderstone. Go to a Pokemon Center to put Pikachu in your party if Pikachu isn't already a party member. Open your inventory and select the Thunderstone. A message appears prompting you to select a pokemon. Choose Pikachu and then watch as Pikachu evolves into a Raichu. Raichu can learn moves like Focus Punch, Giga Impact and Charge Beam.