You can download SIESTA releases from the GitLab releases page.
Information on how to install SIESTA is available on the installation section of the documentation site.
Compiled versions of SIESTA are readily available for download from Conda forge, see Installing Siesta with conda for instructions.
SIESTA became open source in 2016, with the release of SIESTA 4.0. In particular, SIESTA is now released under the GPL-3.0-only license (GNU General Public License, version 3.0 only).