There is no one place.
You will have to poke around a little.
The address PxT gave you above will get you the Linux kernel. Most of the core utilities (commands) are GNU (www.gnu.org).

You can check out the source online via CVS for the *BSD projects.
Look at ftp://ftp.freebsd.org/pub/FreeBSD/de...nt/FreeBSD-CVS to mirror the entire source tree for FreeBSD Unix.

Even better, which Unix are you looking for source to? And why? If you want to compile a full system, why?

Plus, there is no one big ol' source file for ANY unix.