Unfortunately for you, you can't catch Cubone in the wild in the ORAS games.
You could easily go to a trade forum like Bulbagarden and ask for a HA one from somebody.
If you want to get it yourself however, then the best way is to catch a HA Marowak at the Friend Safari in XY and breed it. First you'll need somebody who has a Ground-type Friend Safari with Marowak in it, exchange friend codes, and if the person has already been through the Hall of Fame -- then you and them being online and in game at the same time unlocks the third Pokemon slot in the safari and adds the chance for any of those Pokemon to have a HA. You can read more about this here, if you want to.
And again, finding somebody with the right Safari can be easily achieved by asking about it online.
Here's a link to Bulbagarden's trade forum. They're pretty reliable.
Hope this helped.
