DarkRadiant is open source software, you can download the code from our Git Repository at GitHub. Get yourself a Git client like GitHub Windows and clone the repository at the following URL:
https://github.com/codereader/DarkRadiant.git
Once downloaded, you can compile it following our Compilation Guide on the wiki. There are prefabricated solution files available for Visual Studio 2017 in the tools/msvc folder.
Use OrbWeaver's PPA, or a darkradiant package for Debian, which is usually available after some time in Ubuntu as well.
Download the Flatpak version from Flathub, or search for it in Discover or GNOME Software.
See the Compilation Guide on our Wiki to build DarkRadiant fresh from its sources.