I downloaded the JavaFx source code from http://hg.openjdk.java.net/openjfx/2.2/master/rt/summary.
The name of the folder in the zip file is re-e71070b0e0c0
(Since this is one of the Google hits for "javafx source code", I'll dump this anwer here too):
Quoting from this answer: For Java 8, the source of the public parts of JavaFX are actually laying right along in the JDK installation as a sibling of src.zip: javafx-src.zip - on the root of your normal JDK Java 8 installation.
Eclipse doesn't by default link them up when you add the JRE (or it finds it itself) - obviously a bug - so you'll have to do that yourself, by hitting Preferences -> Installed JREs. Find your JDK 8, hit Edit button. On the list JRE system libraries, expand the jfxrt.jar node, and link in the javafx-src.zip file.