The easiest and most common way to get a 6 IV Ditto in X or Y (excluding hacked Pokémon and trading) is to find someone with a Normal type Friend Safari which has Ditto in it and exchange friend codes. All Pokémon found in Friend Safaris always have at least two perfects IVs but it is still extremely unlikely that you will find a Ditto with 6 perfect IVs.
Good luck.
