The first IV Changer is Located in Superior City. He can, in exchange for an IV Stone, change your Pokémon's IVs to their maximum value of 31. This IV changer is also available as an upgrade in the Secret Base found in Carinae Town.
There is another IV Changer that appears at the very end of the game in one of the houses in the Phoenix Iota. This IV Changer sets your IV(s) to 30 instead of 31, which is extremely useful for getting certain types of Hidden Power