My Petco has a tank set aside for quarantine that runs on a different system cause the dude who runs the fish area knows his stuff. I don't have a problem buying fish from my Petco. I usually get dithers from them.
Usually one whole section runs on one filter and another sections runs on another. So even if one side is sick theirs a chance that the other not connected dont share wAter. You just have to ask. But quarantine is a must regardless.
Ive found there will always be that one petco with better selection than others. I know where i get my oddballs and cichlids others dont bother carrying.