Backports

echo 'deb http://deb.debian.org/debian stretch-backports main contrib non-free' > /etc/apt/sources.list.d/backports.list

apt -t stretch-backports install <package>

Debian 8

deb http://archive.debian.org/debian/ jessie main contrib non-free
deb http://archive.debian.org/debian-security jessie/updates main contrib non-free

Debian 9

deb http://archive.debian.org/debian/ stretch main contrib non-free
deb http://archive.debian.org/debian/ stretch-proposed-updates main contrib non-free
deb http://archive.debian.org/debian-security stretch/updates main contrib non-free