Orator, not to be confused with On the Orator, is a treatise by Cicero, setting out the qualities required by the ideal orator, along with a plan for his education. Cicero singles out Demosthenes as the greatest of Greek models, because of his mastery of the plain, the grand and the middle styles.
Orator at Amazon: United States.
Free online texts
Gutenberg: Cicero's Brutus or History of Famous Orators; also His Orator, or Accomplished Speaker, translated by E. Jones. Multiple formats.
Internet Archive: The orations of Marcus Tullius Cicero Vol. IV, translated by C.D. Yonge (1851). Multiple formats.
Latin Library: Orator. Latin text, HTML format.
Perseus: Orator. Latin text, HTML and XML format.
Wikisource: Orator. Latin text, multiple formats.
History of Philosophy without any gaps: Rhetorical Questions: Cicero - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Raphael Woolf on Cicero - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Cicero.
The Great Conversation: Further reading at Tom's Learning Notes
Cicero: On the Orator.
Tacitus: Dialogue on Orators.
Latin resources: Learn to read Latin texts in the original.