To obtain a Reset Bag in Pokémon X and Y, all you have to do is go to your Super Training Game, on the Home Screen and punch the Default Bag with your Pokémon. After you punch it several times a Random Bag will drop, this could be the Reset Bag or a Bag that will increase the EV's in one of your Stats. You won't get the Reset Bag every time, so if you don't get it first time keep punching the bag until you eventually get a Reset Bag.
If you want more help getting a Reset Bag, check out this video here.
Hope I helped :)