Where can I download the Darwin 12.2.0 source code?
I found it. Actually, the Darwin kernel is named xnu. The complete kernel name, version included, for Mac OS X 10.8.2 is xnu-2050.18.24.

It can be downloaded here: http://www.opensource.apple.com/tarballs/xnu/xnu-2050.18.24.tar.gz.

Source code can be browsed online here: http://www.opensource.apple.com/source/xnu/xnu-2050.18.24/