A great collection of online plays. Many also have access to monologue searching.
Most of our books of monologues have call numbers beginning with PN 2080 and are shelved on the 8th floor of Bobst Library. In BobCat, there is no specific subject heading for scene books. you can perform a keyword search in the catalog for the term monologues. I would also recommend using the Play Index. or one of the other databases listed below, as they can restrict the play by gender, and number of people in the cast.