Here you will find the packages with the current SimpleSAMLphp versions:
Check the GitHub releases page in case you are looking for any other version.
The core library handling the basic SAML stuff (messages, bindings, and so on) is detached from SimpleSAMLphp, and can be found on GitHub:
SimpleSAMLphp itself is also on GitHub, and you will find it here:
Make sure you follow the instructions to install from the repository.
SimpleSAMLphp is available as a Debian package. You can therefore install it with apt-get, but bear in mind that the version available in Debian stable might be outdated. Since Debian backports security fixes, it is fine to use the version you may get with apt-get, but you will miss the latest features.