I want a disk image (for installation). I've checked here and the 'Darwin Technologies' link there but couldn't seem to find anything. I want the Darwin (not any derivatives) and would much rather it from an official Apple source, sorry I'm so picky.
Unfortunately, ISO files are no longer made available. From http://en.wikipedia.org/wiki/Darwin_%28operating_system%29#History:
Up to Darwin 8.0.1, Apple released a binary installer (as an ISO image) after each major Mac OS X release that allowed one to install Darwin on PowerPC and Intel x86 computers as a standalone operating system. Minor updates were released as packages that were installed separately. Darwin is now only available as source code, except for the ARM variant, which has not been released in any form separately from iOS. However, the older versions of Darwin are still available in binary form.