When I was looking champdogs seemed a good place to look and was recommended to me as a good place. I did contact a few breeders on there but then went against most of the advice I'd give people

. If I did it again though I'd pick my breed and look for a breeder who did health tests particular to that breed, usually if you Google the breed and health tests that the parents should've had before breeding. I'm not 100% sure but I remember something in champdogs where if I clicked a breed/breeder it would show some recommendations for what should be health checked.
As you said see the mum. A lot of good breeders may have waiting lists for a puppy so better to contact sooner than later. I've seen some that will instead of letting you go and pick a puppy will match the puppy based on personality. Some will also have contracts to sign and I think it's the same as with cats but may be wrong but if KC registered I found good breeders will put them on the inactive register so if they are bread the pups can't be registered to prevent breeding. I also wouldn't get a puppy unless it has had its first vaccine just to make sure it's been vet checked beforehand. The person I got my dog from asked me if I wanted the vaccine doing before I picked her up because she said some people don't want it doing as they'd rather get a package deal at their own vets. I was just more concerned it would be easy for them to say the pup was vet checked if I didn't get any vaccine record.
I think some breeders will also have a clause to say you can return the puppy within a certain time frame if any health issues so I'd maybe check that and if so get a health check asap before I got too attached.
You could always Google the breed as well as a lot of breeders have their own websites which goes into detail about their breeding