The generated source for the Cog VMs is available from the Squeakvm subversion repository:
svn co http://www.squeakvm.org/svn/squeak/branches/Cog
Building instructions are in the build directories, e.g. build.macos32x86/HowToBuild
You can find unofficial pre-built VMs for both V3 and Spur format Squeak and Newspeak images under http://www.mirandabanda.org/files/Cog/VM/.
You can find bootstrapped Spur images in http://www.mirandabanda.org/files/Cog/SpurImages/.