It depends on the store.
Mine here are good, but I have occasionally seen sick fish. I've only seen 2 fish dead in the entire fish section 1 time. Petco is farther away from me, so I haven't been able to check the fish, but alot of them seem to take care of them well!
I think you'd probably be able to find healthy ones at petco or petsmart.
"Life is an adventure full of beautiful things, and a treasure worth more than gold."