The Guild’s Resource Directory—updated annually—contains a list of agents who specialize in theatre. Additionally, the Association of Authors’ Representatives (www.aar-online.org) has an online database with agents’ information and what kind of authors they represent.
If you are having a production, you should consider inviting agents to attend. If you’re working with a director or designers or actors who like your work, you may try to get her or him to recommend you to their own representatives. You can also be referred to an agent by a theater’s literary manager, artistic director, or by a producer or other theater professional.
You should understand however, that it is difficult for new or emerging writers to obtain representation. There are also opportunities to submit your work to theaters, festivals and contests that do not require an agent or other representation.