A good breeder often has a waiting list for pups before the dam ever gets preggers!
Then once a pregnancy is announced, word of mouth comes into play. Especially if the sire is also a lovely natured, great example of his breed as some potential owners will come via this route too. Previous pups owners may be asked by potential owners who bred their pup.
By good breeder I mean someone who IS responsible in ALL aspects of breeding and has previously produced healthy well-tempered/socialised pups but doesn't produce a litter too often. Doing it properly is hard work.
With the prevalence of puppy farming - the really caring non-profit making breeders often sell most pups totally by word of mouth or personal recomendation. They also won't produce a litter in the first place unless they are 100% positive good homes can be found for all the litter - it's this factor that has led to the decline in some breeds like the otterhound, as general demand dissiapated amongst the public.
I also know a few people who have stopped breeding staffs in the last few years, as they were upset by the numbers clogging up the pounds and didn't want to risk adding to the problem. How this will affect the staff when they fall out of fashion again I don't know. Many owners of poodle studs are refusing to participate in the current doodle craze too. A responsible breeder is usually mindful of the welfare of their chosen breed as a whole and does it for love not £££££
In general having a chat with the relevant breed club is likely to put you in touch with responsible breeders who are considering producing a litter.
I personally think that as a dog is a long term thing it's well worth waiting for the right animal to come along. Then again I may be biased as my last dog finally slipped away in his sleep at 18 lol!