Neal its a bad idea to buy puppies from pet shops. These shops most of the time sell puppies bread by back yard breaders. Problems arise from these puppies, which can be genetic, behavioural, parasitic or bad health. It is always advisable to study the breed that you want either on net or visit nearest kennel club for details. Once you have identified the breed that you want look for responsible breeders of that breed from net or from kennel clubs or local vets. Always insist on seeing the parents of the puppy first, this is very important. Look at the complete litter and then select the puppy that you like, remember that the puppy should be active and self feeding. Have the puppy checked by a vet and then buy it.
Remember a puppy will be with you for a life time and will be like you kid all their life, so take some time out, do some serious research before buying the puppy.