To get Raichu in Pokemon Yellow you have to trade with someone who has a Pokemon Blue/Yellow game. Trade your Pikachu to them. If you have noticed in Pokemon Yellow, Pikachu refuses to take Thunder Stone. When Pikachu is traded to Blue/Red game, give Pikachu the Thunder Stone, Pikachu will now except and evolve. Then trade Pikachu back to Yellow. You will now have a Raichu for Pokemon Yellow!