The package is provided by the AUR. You can install it with an AUR helper of your choice: system-tar-and-restore or manually by invoking the following commands as a regular user. (to build packages from the AUR, the base-devel package group is assumed to be installed)
wget https://aur.archlinux.org/packages/sy/system-tar-and-restore/system-tar-and-restore.tar.gz
tar xf system-tar-and-restore.tar.gz
cd system-tar-and-restore
makepkg -si