If you're very discouraged by your pet store Bettas dying, definitely look into other venues for buying your fish ! Just about every state has at least one Betta club, so search for your local club and ask them about local auctions and meetings, or ask private breeders from the local club about buying a young Betta directly from them. A Betta from a breeder is MUCH more likely to lead a healthy, long life... you'll get the Betta when it's 3-4 months old and healthy, as opposed to a 6-12 month old Wal*Mart Betta with fin rot, parasites, and having been through strenuous shipping and living in a tiny cup for months. When you buy a Betta from a good breeder, you also know you aren't supporting how Wal*Mart and Petco neglectfully treat their Bettas.

You could also call around to local aquarium stores to ask if they have Bettas. Most fish stores don't keep their Bettas in cups dying from disease and ammonia, they NEED to have a nice presentation for fish owners so they're much more likey to keep their fish clean and healthy and buy from local breeders.

Also, make sure you check to be sure that you aren't doing anything to shorten the lifespan of your fish. How big is your tank? Anything under 5 gallons should be changed 100% every week. Is your water PH at a safe level? Do you use dechlorinator? Just make sure you have everything down