The Dialogue on Orators (Latin: Dialogus de oratoribus) is a work on rhetoric attributed to Tacitus. Although this has been questioned, the concluding speech by Maternus accepting the necessity of the empire in spite of the resulting decline of oratory seems in keeping with the tone of Tacitus' other work.
Wikisource: Dialogue on Orators, translated by Alfred John Church and William Jackson Brodribb, 1876. Online, downloadable as PDF/MOBI/EPUB
Librivox: A Dialogue Concerning Oratory - Public domain audiobook.
Wikipedia: Dialogus de Oratoribus.
The Great conversation: further reading at Tom's Learning Notes.
Tacitus: The Annals.
Tacitus: The Histories.
Tacitus: The Agricola - presents its subject as an example of ho to deal with the circumscribed opportunities of public life in the empire.
Tacitus: The Germania.
Latin Resources: Online materials for learning Latin.