I would recommend people to go the breed club web site of whatever breed they are interested in, to contact the secretary and ask whether they have a rescue scheme, or a list of reputable breeders with puppies.
There are many puppy sales sites, but you have no way of knowing if the sellers are genuine, and whether they have any real interest in the breed or the puppies they are producing.
A lot of people use a site called epupz, but it is a case of 'caveat emptor'. Epupz do have some guidelines for buyers
but I know some very dodgy breeders who have advertised on there and Joe Public would have no way of knowing.