There are some very disturbing videos on Youtube showing the poor conditions wal-marts fish are kept in.
One of them shows a tank, filled with dead fish just sitting there.
And the bettas are just left to sit in cups.
Luckily, in the Uk, supermarkets don't sell fish. ASDA does sell horrible little goldfish bowl kits but I think almost all supermarkets and pet stores do.
My local fish store is very good, the bettas are displayed in large tanks with Platies, and the employees are very helpful and they always ask a few questions about your tank before they let you buy fish.