Using the first example on the list of https sites:
#deb http://ftp.debian.org/debian stretch main contrib non-free
#deb http://ftp.debian.org/debian/ stretch-updates main contrib non-free
#deb http://security.debian.org/ stretch/updates main contrib non-free
deb https://ftp.tu-graz.ac.at/mirror/debian/ stretch main contrib non-free
deb https://ftp.tu-graz.ac.at/mirror/debian/ stretch-updates main contrib non-free
deb http://security.debian.org/ stretch/updates main contrib non-free
deb https://ftp.fau.de/osmc/osmc/apt/ stretch main
#deb http://apt.osmc.tv stretch main