I like to search up on the internet who the winners of the recent nationals and sweepstakes for that specific breed were and contact them. The national breed clu websites also often have lists of members which is a good place to look as well
My own state (PA) has a statewide classified that used to be called "pennswoods" but now goes by Rennug. It's small, but excellent. I definitely recommend peeking around in your state to see if you can find one like that. Local facebook groups are also great for keeping tabs on other people working the same breeds. I'm in one for my city, and for the general region of the state I live in. Hoobly is another good free classifieds platform, but it is more active in some states than others.
Well I pretty commonly use RabbitBreeders.us and searching through people's Facebook pages. RabbitBreeders.us is the largest rabbit directory in the country (or maybe the world I forgot) so you have a lot of people to choose from.