Really? Where can you find the source code? I never noticed it on their site - seems like they're a typically closed-source company to me. Am I missing something obvious?
Second way, and probably the best, would be to go to its store page, and in the URL will be the extension ID; for Ghostery, it's "mlomiejdfkolichcflejclcbmpeaniij". Now, just find that in your Chrome installation directory, e.g. for Win7 the extension would be located at %LOCALAPPDATA%\Google\Chrome\User Data\Default\Extensions\mlomiejdfkolichcflejclcbmpeaniij\.
The last way would be to intercept or otherwise directly down the .crx file (curl, wget, etc.), which is just a compressed zip archive. You can do the same for FF extensions as well, those .xpi files are just zip files.
They're also an ad (analytics?) company. I just don't trust them.