Odd Egg: From the Day-Care Man, will hatch into a Pichu, Cleffa, Igglybuff, Tyrogue, Smoochum, Elekid, or Magby that knows Dizzy Punch. It has a 14% chance of being Shiny in international versions. In the Japanese version, it is available by trading in the Egg Ticket from the Day-Care Man at the Pokémon Communication Center in Goldenrod City and has a 50% chance of being Shiny.-Source
You get a Odd Egg from the Daycare man which has a chance to be Pichu. Thats the only way to get a pichu without breeding it seems.