You have to get in before litters are due. Many breeders will have ideas of when their next litter is going to be many months before the bitch is pregnant - though obviously subject to biology.
I did it like this:
Took a list of all the breeders on the KC list that looked like they were active.
Contacted those within 150 miles of me with an introductory email in which I explained a bit about me, my dog experience, what I was looking for and when. Ask them to keep me in mind if they were planning.
Many responded to say they were/were not going to have litters in the time scale. Those that said they were I researched further online - basically gathering any information I could to check they were who they said they were, looking for details of previous litters, checking the health tests of their dogs and so on. I cut out anyone I had doubts about.
This gave me a shortlist of breeders who were planning and who I THOUGHT I might be happy to take a puppy from.
I kept in touch with them to get updates as we got closer to the date and when one conformed they had a litter I went to see them. We had a long chat in which we both interrogated each other for about 2 hours. I got to see the litter, meet the mum and dad, see the sale contract, ask a million questions. We both went away for a think and confirmed to each other we were happy to go ahead.
From the initial list to getting a puppy took about 7-8 months.
There are variations on the process above but you will do better getting in before litters are born - especially for bitches which I would expect to be sold first as they tend to be the most popular.