Ask your performer friends which agent they are with and see if they can put in a word for you with their agent, or refer you to another reputable agent. Get involved with activities that will put you in touch with other performers, such as workshops, membership meetings, casting showcases, and special seminars. Agents want experience. Many agents will consider taking on new clients only if they have a strong theatrical background. Get involved in a play or showcase and send invitations to your targeted agents.
Many agents periodically attend plays and showcases to scout new talent.
Even without an agent, there are things you can do to get jobs on your own. Check the trades for casting calls, and contact casting directors directly. All resumes are not created equal. Always keep your resume current, and remember to include all union affiliations. Submit appropriate photos.