As the site rules do not allow the promotion of individual breeders anywhere on boxerworld, nobody is able to give you a specific recommendation here. You would need to do the research to find a responsible breeder yourself
We can, however, suggest that the best place to start such a search is through your nearest boxer club - or all breeds club, if there isn't a boxer club near you. A breed club should be able to point you in the direction of any breeder members who have litters coming up. Another good place to find breeders of good quality dogs is, of course, dog shows. One thing you should check carefully about is the screening any breeder performs on their breeding stock to ensure that they're not breeding from animals that carry any of the genetically inheritable health problems common to boxers. You can read more about the conditions that good breeders MUST screen breeding animals for on this page:
Boxer health testing - which also has a link to the American Boxer Club's recommended screening for all boxers used for breeding.
For finding your nearest boxer club, try this link:
U.S. Boxer Clubs