Where can I find the latest release of Darwin?

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,[4] 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.

[4] http://lists.apple.com/archives/Darwinos-users/2007/Oct/msg00006.html

The older (8 and earlier) releases were released as binary images. Some are available for download at this Apple site.

See this mailing list post for more information.