Add contrib non free to sources:
$ sudo nano /etc/apt/sources.list
deb http://ftp.us.debian.org/debian/ stretch main contrib non-free
deb-src http://ftp.us.debian.org/debian/ stretch main contrib non-free
deb http://security.debian.org/debian-security stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security stretch/updates main contrib non-free
# stretch-updates, previously known as 'volatile'
deb http://ftp.us.debian.org/debian/ stretch-updates main contrib non-free
deb-src http://ftp.us.debian.org/debian/ stretch-updates main contrib non-free
$ sudo apt-get update
$ sudo apt-get install unrar
...
Des:1 http://ftp.debian.org/debian stretch/non-free armhf unrar armhf 1:5.3.2-1+
...
$ unrar -V