Download short individual scripts online. Brooklyn Publishers and JD Drama Publishing both offer a decent selection of duet pieces -- male/female, male/male and female/female -- for a reasonable fee, usually $10 to $15. Both sites allow you to read a sample before purchasing.
Buy, or loan from the library, a collection of short duet pieces to see if you can find one that works for you. Smith & Kraus, a publisher that specializes in works for the stage, has a four-volume Duo Series. Contemporary Drama Service also has a number of titles from which to choose.
Select a scene from a full-length play. Try to find a scene that stands on its own. Shakespeare is an excellent source for scenes with two actors, and his plays are both widely known and widely available. Moliere is good if you are looking for comic material. For something more contemporary, try a scene from Beckett's "Waiting for Godot."