You can get Hitmonlee and Hitmonchan in the Fighting Dojo on Saffron City. But first, you must fight Karate Master Kiyo/Koichi. They use Fighting Type Pokemon. But remember, you can only choose one Pokemon.
You get the "Hitmon" Pokemon by beating the Fighting Dojo in Saffron City.
Note: You can only pick 1 Pokemon, Hitmonlee or Hitmonchan.
You can get the other by getting the National Dex and then breeding into Tyrogue.