I'm not sure how they would be obtained. Maybe you would find them in arena chests, or maybe Sports Fans would randomly drop one on defeat 25% of the time, maybe you could get one from a PokeStop, perhaps some Fighting-type Pokemon could drop them, or perhaps they could be bought from a particular shopkeeper.
In order to use the punching bags, you would first place one on the ground. Then you would throw your Pokemon's Poke Ball at the punching bag. Your Pokemon would pop out of its Ball and you would watch the Super Training bag start to become mined on its own. When the bag breaks, your Pokemon gets the effect of the bag.