Here's a small sample of the configuration file that contains the repository list where apt can download .deb files.
The following example is a source list mostly of italian mirrors.
eva:~# cat /etc/apt/sources.list
deb http://security.debian.org/ stable/updates main
deb ftp://debian.fastweb.it/debian/ testing main contrib non-free
deb-src ftp://debian.fastweb.it/debian/ testing main
You can find best mirror for you here : http://www.debian.org/mirror/mirrors_full .
If you cannot find best solution, netselect tools (http://packages.debian.org/unstable/net/netselect-apt) help you to find mirror with fastest response times.
How software is installed, updated and managed on different Linux distributions