Days late, but the sources can be found in a few different places depending on what you’re looking for.

The “official” Plan 9 (a.k.a. 4th Edition from Bell Labs) as controlled by the Foundation can be found here.

After that there are a few different “development” branches, so to speak. There’s 9legacy, which is basically Plan 9 with some patches for modern hardware, and then there’s 9front, which is where much of the real development of Plan 9 actually occurs. Others also exist in varying states of development/decay — 9atom, harvey, and r9, mainly.

9front is where most of the action is these days, but it’s not “official” in that it isn’t directly controlled by the foundation. However, at least one member of the foundation (Ori Bernstein) is a primary contributor and developer.