I had bought a betta next door at one of the box-store Animal places. Within 4 hours it died.
Took it back the next day, got my money and headed down the street to Age of Aquariums.
HEALTHY FISH were to be found. Not half-dead floaters polluting the tanks. Oh no. And there wasn't the obligatory fish-store funk smell in the air, either.
The staff is very knowledgeable about fish and won't let you buy conflicting species. They question you to make sure you know what you're getting into if you buy a particularly difficult species, or whether your tank can hold that many fish if you happen to buy alot of fish at once.
Don't be fooled by the "all in one" gigantic pet stores. Go to smaller, locally owned ones and you'll not only get healthier fish (The fish I bought there lived for nearly 5 years), but you'll be supporting small business owners, who always need our help