Champdogs is reputable, check their website, their breeders have to be KC registered and they show the full pedigree history for the bitch/sire and health scores. It will give you a good description of the breeds you are looking at, plus a list of breeders. There is also a detailed section about what to look for when buying a puppy.
A good breeder will interview you as much as you want to find out about them and will insist on vetting you in person before offering you a puppy. They will not offer you a puppy on payment of a deposit to secure via email/phone. They should only really be breeding to gain a puppy for themselves, there's really little reason why they would breed otherwise. And they should have the welfare of their breed as priority.
They should offer lifetime support and advice and be prepared to take the dog back at any time in the future, rather than it go to rescue. All appropriate checks should be done for the breed.
Make sure you see all the relevant paperwork. Most breeders will worm and generally now microchip too now (both of ours have anyway)
Don't buy from preloved etc.
Small things make a difference, our first breeder has our dog if we go away, so he has only ever known our house and theirs. Our latest breeder is going to show DH how to work the new puppy (a Cocker).
Generally, just do your research about the breed and the breeder.