There are a variety of ways to get a plumbing apprenticeship, one being to join a local plumber's union. This allows you to become acquainted with some of the plumbers in your area and find someone with whom you feel comfortable working with.
Another option is to make a resume listing your qualifications and pass it out to local plumbing agencies. Remind the companies who receive your resume to keep them on file even if they are not currently hiring. Finally, try attending a two-year trade school or career center to improve your plumbing skills and make new connections within the field.