Googling "wpilib source" gives
https://usfirst.collab.net/sf/projects/wpilib/
as one of the first few results. Then, under the wiki, you can find directions to install the eclipse plugins. The address that is hyperlinked is incorrect, but the text "http://frcbuilder.wpi.edu:8190/view/2015/job/eclipse-plugins/lastSuccessfulBuild/artifact/edu.wpi.first.wpilib.plugins.updatesite/target/site/" is correct.
Installing the plugins puts wpilib at C:\users\[your user]\wpilib\java\2014.03.267\lib\Wpilib-sources.jar. You can use winRar to extract the source code.
Exploring other pages on the site brings you to lost of other good stuff too, like
http://frcbuilder.wpi.edu:8190/view/...na/ws/WPILibJ/
which looks like what you're looking for.