You might try the eBooks section of
this site, there are several complete Bibles and New Testaments.
https://www.mobileread.com/forums/ebooks.php
http://www.gutenberg.org/etext/26270
I am assuming you mean Jesus' speech as recorded in The Gospel of Matthew Chapters 5-7, a quick Google(TM) search for "Sermon on the Mount" eBook brought up some books by that title, not all free.
http://www.google.com/search?q=%22Se...&start=10&sa=N