A good way to locate a reputable breeder is to find one through the English Springer Spaniel Field Trial Association, the American Kennel Club or a regional breed club. Both the ESSFTA and the AKC have online breeder listings sorted by state to find local breeders.
If you have an existing relationship with a good veterinarian, consider asking her for recommendations. Visit local dog shows and field trials as well, and ask any friends who own springer spaniels where they got their dogs. However, avoid casual or backyard breeders who do not show or health test their dogs.
Once you have located potential breeders, spend some time talking to them and researching them. Try to visit the facility, and see how the dogs are kept and handled. Avoid breeders who have puppies available at all times or who breed their females more than once a year. Both parents should also be at least 2 years old before they produce their first litter. Springer spaniels do have hereditary health problems, so a good breeder health tests both parents and is willing to provide proof.
A good breeder should also ask you questions about your lifestyle and plans for the dog and then match you with the right puppy. Good breeders often temperament test their puppies to aid this process. The puppies should also be socialized and handled regularly to give them good starts.