Raichu is a stone evolved Pokemon. Stone evolved Pokemon are NEVER available to catch in the wild (in the handheld games). You can sometimes catch their pre-evolution (but not in Yellow). To get a Raichu, feed a thunder stone to your Pikachu, which like I mentioned is NOT available to catch in the wild either. In Pokemon Yellow Special Pikachu edition, Prof. Oak gives you a Pikachu when you start. However, be warned:
- Once you evolve Pikachu into Raichu, he will not follow you around in the game anymore. He will be just like any other Pokemon in your party.
- You can not de-evolve Pokemon. Once you feed the thunder stone to Pikachu, the change is irreversible.
- The special upgrade to a "Pikablu" you get from Pokemon Stadium is only available to your Pikachu starter, not Raichu!
- To get a Pikablu (a surfing Pikachu) follow these instructions: http://faqs.ign.com/articles/588/588815p1.html
This does not apply to the other first generation Pokemon games, in which you could catch Pikachu in the wild (Red, Blue and Green). However in these games (equally) Pikachu will not follow you, or wont be upgraded to surfing Pikachu. That feature was only available in Yellow (that's why they call it "Special Pikachu Edition") even though there is no other version of Pokemon Yellow :)