He's in shaking grass patches. There's about a 10% chance of finding him, 10% each of the other monkeys, and 5% chance of the evolved form of whatever grass Pokemon is in your game (the cotton one or the bulb). The rest are Audino.
Shaking Grass. Even then, it's only a 10% chance of finding him or one of the other monkeys. Audino appears for 65%, while the remaining 5% is either Whimiscott or Liligant, depending on what version you have.