Plays and novels published prior to 1923 are in the public domain and may be freely adapted without obtaining permission.
Works published more recently than that vary in their status. If the published material you are interested in adapting is not public domain, you will have to option the rights to it, usually involving a payment. You should be able to search out the author's representative and ask about the feasibility of doing so.
Although Argo was adapted from a portion of the main character's book and a magazine article, the facts of the story are actually public domain. Had the screenwriter wished to do so, he could have written a screenplay based on that incident without optioning material. By optioning the material, however, he was able to avail himself of much more detailed information directly from Tony Mendez - who experienced the events firsthand - as well as the research conducted by the author of the magazine article.