We actually read the full speech in school and watched the youtube video. We got it as a copy from a textbook called "Speeches that changed the world" by Simon Sebag Montefiore.

You can get the ebook from the google play store. From the free sample it seems like they got permission to reproduce copyrigth material from The Estate of Dr Martin Luther King, Jr.

https://play.google.com/store/books/details?id=jPArcMn728kC