Collapse OS is distributed as a source tarball which is updated regularly, the latest of which is available here. A list of all available tarballs is here. The changelog included in every release can also be peeked at directly here.
Collapse OS used to be published as a git repository. An archive of this repository is available here. Note that this repository also contains the first incarnation of Collapse OS entirely in z80 assembly. Checkout the "z80asm" branch to access it.
As an experimentation, a uxn port of Collapse OS has been created to see if the fit was good, if there was an opportunity for some kind of cross-pollination to do with it.
It was interesting to do, gave me good ideas, but the fit wasn't good. The port is functional, but will not be kept in the Collapse OS snapshots. It might be of interest to some people though, so it's available here. See emul/uxn/README.
The follow-up effort by Hundred Rabbits lives here.
At the moment, this website is served under both HTTP and HTTPS, but I'm planning on phasing SSL out in a near future. That might make you worried about a man-in-the-middle attack on the archive you're downloading here. Sure, it can happen, but considering that:
You are advised to manually check the code before building it. Then, once you've checked it, you can diff that known-good archive against any new snapshot you download for quick malicious code detection.