Unfortunately there is only one way to find a Grip Claw in X and Y, which is finding it on wild Sneasel, who have a 50% chance of holding one. Besides this, there is no way to find a Grip Claw, so the process may end up being long and repetitive.
Wild Sneasel can be found on Route 17. There are multiple ways to know if it has a Grip Claw or not; some include leading with a Pokemon with Frisk, or using Thief to steal the item if it is holding it.
Remember, Sneasel has a 50% chance of holding the Grip Claw, so you aren't guaranteed to find one every time. Wild Sneasel also have a 5% chance of holding a Quick Claw, so don't get confused with that.