I wouldn't ever recommend pet shops, places with more than 3 breeds/litters for sale and places that won't let you see the living conditions, the parents/other puppies in the litter, the vet papers etc. Dogs from pet shops are most likely from puppy farms/mills and they are likely to get sick and die pretty quickly. Even if you feel sorry for a dog in a pet shop, don't adopt it because the chances are it came from a puppy farm/mill and your money would be supporting the bad breeding. Watch out for signs of sickness, like bald patches, coughing, runny eyes and noses, etc. and make sure they are living in a house with things they would see every day in another house.
I have a book by Dogs Trust with a small section with advice on where to adopt a dog from and how to avoid puppy farms. This is all info taken from that section but in my own words. Sometimes breeders vaccinate the dogs before they're adopted so it saves the new owners a job :)