Or you can call your local humane's society.
I did that on a petsmart once. I was pissed at this store b/c i had bought a (unknowingly) sick gecko for about $40-80 from them, and it died within 2 weeks (wasn't my fault, it was malnourished and sick, two things i didn't know at the time. I think it had "Crypto"). They did refund me the money, but I checked out the cage as I was leaving and everything had gone to "poop." There was droppings all over the cage and not a drop of water or moister in the gecko cage. Their temps were about 100 degrees which was about 10-15 degrees to high.
They usually send a specialist *the humane's society* (reptile specialist, fish, ect ect) and they'll assest if there's a case of abuse. If there is the store usually is fined and in turn the store fires who-ever is suppose to be in charge of that dept. for not supervising that area properly. Usually they'll fly right after that.
I went back to the petsmart that I called the human's society on about a month after i made the call. They had a new reptile guy who knew his stuff, the temperatures were perfect and everyone looked well fed (before they were malnourished. they probably culled the sick ones, cleaned the cage and brought in a new batch)
It sucks, but its a reality. Know as much as you can know about a pet and then give it the best home you can. I wish more things can be done for bettas, but I also have 4 rescue cats (i think 2 of them were abused previously) and they're higher up on my priority than my two bettas. That's just reality for me. Be nasty to a dog (despite that I hate dogs) or a cat and I'll get to be a real jerk, but with a fish it sucks, and all you can do is call the local animal "cops" per-say and lets them know whats up.