Tarballs for the mingw-w64 sources are hosted on SourceForge. The latest version from the 4.x series is 4.0.0. The latest version from the 3.x series is 3.3.0. Winpthreads has been merged into the main tarball as of 3.1.0.
Details on how to get the mingw-w64 code from Git and an Git-web viewer are available on SourceForge.