Incidentally, please do take the 17 minutes to watch this speech (and perhaps follow along with yread's transcript link). It really is one of the greatest speeches ever given.
Also keep in mind that the last portion starting with "I have a dream..." and following was all improvised. The original draft didn't include those words.
The distribution of the printed text is far less controlled than the video, which is much harder to find (legally) than the text.
In the case of the text, it is possible to find copies online (for, via the National Archives), but redistribution in other printed works (namely, textbooks) is problematic.
As for the video, the King estate controls that very, very tightly.