All my clown loaches and my original marble angel, the other one I just euthanized after over 2 months of treatment for swim bladder, were purchased from either Petsmart or Petco. My ottos in my other tanks were purchased from Petco, so I have no problems with them. All the fish I bought from either stores are still alive, since my many years in the hobby.
Basically, with any place you buy fish from, you do the following:
1. Buy healthy fish from a healthy tank
2. Never buy fish from the same tank a sick fish is in.
3. Watch the fish in the tank for a few days for any illness
4. Find out when they receive shipment of their fish. Then wait a few days when new fish arrive from shipment, in order for the fish to acclimate properly.
5. Acclimate your fish properly
You follow those, then I can guarantee you will have nice healthy fish you take home.