How can I get the source files for Arduino libraries?

The code is also available online for those times you don't have the IDE installed:

https://github.com/arduino/Arduino

The base Arduino source files are in the Arduino installation, under cores/arduino/. Source for additional libraries included are in libraries/.

On a Mac, the path to the source code folder is:
Arduino.app/Contents/Resources/Java/Hardware/Arduino/Cores/Arduino. I'd expect it would be the same from /Java/ on down in other OSes.