I have searched all around google for the originial source for the last release of Darwin OS that apple release. What I found was different OSes such as GNU Darwin and pureDarwin that were based on the Darwin Source code. Is there any place I can fork the original Darwin Source Code, or does it simply extinct?
Check Apple's open source site. The latest (10.7.4) kernel is xnu-1699.26.8, other source packages for 10.7.4 are available too. Remember that not all parts of Mac OS X are open source. You will not be able to build the complete system by source.
Apple provides information about how to build and debug a kernel in their developer library. Besides the devolper library you may find the book Mac OS X Internals. A Systems Approach. by Amit Singh useful.