How to find the script (screenplay) of old silent movies?

To answer the first part of your question, there are a couple of decent resources for old movie scripts, Simply Scripts being a good one.

As for Birth of a Nation, it was my understanding that Griffith did not commit a script Per Se to paper, but rather had the whole film in his head at the time of shooting. The interstitial cards were added in retrospect to convey emotional moments, inner thoughts and extra exposition.