Even LFS will have dead fish in the tank. It really all just depends on how often they check the tanks. Of course, LFS do tend to care more about their fish and have less repeats of sickness, but its always possible.
I only buy Betta from Petco, why? They have so many kinds and they are still young. They are in clear water, in light and easy to get to. They also have lids on them.
I buy all other fish from Petsmart. Then again, I'm pretty much done buying fish. Maybe Goldfish in the future. Their Betta are in blue water, dark shelves, and are very hard to get too, they also do not have lids. They tend to be big, old fish in there too.
I have bought sick fish from both stores, they had Ich, while my tank also had Ich. They all recovered just fine and no losses. But I really wouldn't buy from either when you see any fish that is sick until at least a week after no more signs. Of course, if your willing to risk it and medicate the fish, just go for it. You can always return it later.