Aitareya Upanishad is one of the Upanishads in the Rigveda. Here you can download Aitareya Upanishad PDF document for free. It figures at number 8 in the Muktika canon of 108 Upanishads.
Aitareya Upanishad is one of the great Upanishaths upon which the great saints like Aadi Shankaracharya and Madhvacharya are commented.