If the mirror hostings are not available in source list, you can always add one.
First, to be on safe side, backup the sources.list before editing it.
sudo mv /etc/apt/sources.list /etc/apt/sources.list.old sudo vi /etc/apt/sources.list
The entries in the file follows like : type, location and components of a repository,
Here is what my sources.list look like after modifying.
deb http://ubuntu.ntc.net.np/ubuntu/ quantal main restricted multiverse deb-src http://ubuntu.ntc.net.np/ubuntu/ quantal main deb http://ubuntu.ntc.net.np/ubuntu/ quantal-updates main restricted multiverse deb-src http://ubuntu.ntc.net.np/ubuntu/ quantal-updates main deb http://ubuntu.ntc.net.np/ubuntu/ quantal universe deb-src http://ubuntu.ntc.net.np/ubuntu/ quantal universe deb http://ubuntu.ntc.net.np/ubuntu/ quantal-updates universe deb-src http://ubuntu.ntc.net.np/ubuntu/ quantal-updates universe deb http://ubuntu.ntc.net.np/ubuntu/ quantal-security main restricted multiverse deb-src http://ubuntu.ntc.net.np/ubuntu/ quantal-security main deb http://ubuntu.ntc.net.np/ubuntu/ quantal-security universe deb-src http://ubuntu.ntc.net.np/ubuntu/ quantal-security universe
deb or deb-src, indicates the type of archive: whether it contains binary packages (deb), that is, the pre-compiled packages that we normally use, or source packages (deb-src).
http://ubuntu.ntc.net.np/ubuntu/ is the local repository hosted by NT, where I can find lots of packages necessary for apt-get. It may differ based on your location.
Make sure to run apt-get update after editing the /etc/apt/sources.list, APT will obtain the package lists from the sources mentioned.