I forget how we got this... I think they were either restockable or you got them from kicking the old vending machine or something...
Anyway inside the sport bags you could find well nothing too amazing most times, it was always a gamble. But at rare items you could get a gym ball which was a stat booster.
My idea is... what if you could randomly get a sport bag from the training center? Like don't make it super easy, but rather a finite chance of getting one after taking your pet from their training course. It would make sense... or at the very least what if Jim from the training center sold them but you could only get 1-2 a day, you're still gambling with opening the bags.
Well I apologize if this made no sense. Or if it did and you have anything to add, feel free. For what it's worth, the old gym balls weren't game breaking, I think they had a slim chance of actually boosting stats so again another gamble.
[edit] Sorry fixed my post. They were SPORT bags. Not gym.
Training sounds like a good way to possibly get them. How about as a rare find at the Bathhouse on certain days? I could see some sport bags being left there by accident.