I don't think you can get Pichu anywhere without breeding two Pikachu...they certainly do not live in trees anywhere to my knowledge.
Maybe you are talking about the
cancer Spiked-Eared Pichu you can get from there? To get that you need to bring a shiny Pichu given out from an event a year or two ago (called the Pikachu-coloured Pichu if you want to look for it via trading) to the shrine in Ilex forest. (Note you cannot evolve that Pichu or trade it, and if you just want any Pichu that defeats the purpose of getting a 'special' Pichu since you need another special one to get that one!). Just try getting wild Pikachu (in Viridian Forest or Pokewalker courses from memory) and breeding them for a Pichu via the Day Care south of Goldenrod (route 34).
If you have any other questions, ask them in the Help thread sticky next time please. Thread closed.