You could start by looking at American Boxer Club, they Might reluctantly put you onto a litter.. But you still have to do your Home work. I remember there was a best of breed Westminster winner, that I would never breed my line into.
History, health, homework.
Many breeders have both sexes in their home. My primary stud dog lives with us and 3 other females. I have several co-owned girls/boys placed in other homes but there is a possibility that the parents of a litter both live with me.
Not all breeders want to deal with the politics of the show world.
It doesn’t take 3 years to health test and properly match a breeding pair. In the US 24 months is recommend where as in Canada it’s 18 months however many wait until 24 months.
When seeking out a breeder, have all your questions written out. Decide what is important for you in a breeder. Make sure your breeder health tests for at least hips, heart and DM. Thyroid testing is a bonus.
Actually meet your breeder face to face. See how they live, and where the dogs are kept and raised. Run their name through google and see if you can find any reviews good/bad. Follow your gut!