Pixielator - In the past when I would find bad conditions at the store I would go to the nearest register or find an employee and ask to speak with the manager about their conditions. Speaking to the manager directly is important, as this thread has proven. They are the ones who will make sure things are improved and have a say in not charging for the fish. Be prepared for a bit of wait if it is a Walmart store. Not sure if they are just super busy or they hope you leave. I've waited up to half an hour. When the manager does show up I point of the problems calmly, politely, but firmly. I'm a very shy person with a social phobia, but seeing animals suffering brings out a confident side of me.
Try not to sound judgmental. I also will typically say, "I know they are just fish, but this is bad buisness."
In my experience, the manger is usually very apologetic. After showing the poor conditions and explaining the illnesses the fish have I will show the fish I hope to rescue. I tell the manager that this one is sick as well, but I feel that there is a chance I can save the fish. I explain the fact I have a variety of medications on hand along with the knowledge needed to give him a fighting chance. Usually the manager doesn't have a problem giving the betta free of charge after all that. They probably just want you out of the store, haha.
Erika | Betta Fish Advocate