if you live in the cali in the high desert....there is a 99cent store that sells betas in a tiny little cup that has only about one inch water and that little bit of water is nasty brown you can hardly see the fish. this is how i got my first beta, i rescue them from there as often as possible. help me get save them if you live near. i have 7betas and 3 trutles so you can imagine all the tanks in my house. my mom says no more but they still need rescuing...
If you are buying the fish, you are not really trying to help diminish this trend. Buying them this way simply helps their trend to fluorish. Only one way to save them, is unfortunately to boycott the store completely and spread the word to others nearby about the cruelty involved.
maybe you'd do better if you talked to the store's management? or perhaps had a petition signed by the neighborhood residents that says if they dont' start treating their bettas with a minimal amount of care, they'll all boycott the store. You can also post signs on the front of the store right after it closes that say things like "fish abuse" or anything else you want to say. surely they'll remove them in the morning, but lots of people will see them during closed hours and maybe they'll get the point eventually. sadly, i also believe that buying them (or maybe even just taking them) but not making a case for why you're trying to rescue them will only make them think it's ok to keep their animals that way because people accept it and buy them anyway.
good luck on your fishy crusade. i admire your desire to help them all.