To obtain a reset bag in XY, simply spam-punch the default bag in the (Core) Super Training Regimen. Although, you can get a Reset Bag by doing this method, you may receive other bags such as the Attack Bag M, so be patient.
An easier way to obtain a Reset Bag from Core Training is by simply waking up a sleeping Pokemon in the training regimen. Since the Pokemon wakes up, it gains more energy, so there will be a higher chance of Core Training bags to appear. I was able to get 4 Reset Bags from waking up a sleeping Pokemon in less than minute.
Team Flare bags can help boost the energy of your Pokemon, too.