Bettas are pretty hardy. My first guess is that it could be your water. If you have a high chlorine content, even half fresh water could do them in. I use a product called Start Right. It removes the chlorine and chloramine in the water.
Here is website that might help you as well:
I'm not a big fan of the WalMart live pet dept....they really aren't any more expensive at places such as PetCo and PetSmart. And you may even have more of a choice in type and colors.
Up until Hurricane Ike came along, the Betta I had was over 3 yrs old. Then Ike hit and we had no electricity for 10 days and I lost my whole tank full of fish
I'm hoping the Betta I have now will do as well as the last!