This depends on the nature of the disability the dog is being trained to support. For example, people with sight issues should contact the Guide Dogs for the Blind organization, who run programs to spot talented puppies with the potential to be service dogs. Other organizations linked to specific disabilities can put you in touch with trainers registered with the specific skills to train a support dog. Alternatively, professional bodies such as the Association of Pet Dog Trainers provide links to useful resources that may be of help.