Pichu actually evolves from Pikachu based on Friendship. Type friendship/happiness in the search bar above in order to find out ways to raise your friendship. Pikachu is the one who needs the Thunderstone, in order to evolve into Raichu.
You can find a Thunderstone in the Underground. Just keep on digging and hopefully you end up finding one. A sure way to get a Thunderstone is in Sunyshore City. There is a Thunderstone behind the Lighthouse.
You can also have someone have their Pokemon hold a Thunderstone and trade it to you.