Yes it is in the rock tunnel, Mt moon, and victory Rd, but the longer you train a pokemon the better. The onix you will find in victory road are about level forty six I believe. This does not give much time to train it. If you catch onix early on you won't need a another rock or ground type because yours will be stronger than anything else you can catch. At least of the rock type pokemon so catch it early on in Mt moon if possible, but it is much easier to find in the rock tunnel and victory rd.