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: