Since apt-get is automatically fetching packages for you, it must know something about where to find packages that haven't yet been installed. This knowledge is kept in /etc/apt/sources.list. Although you can edit this file by hand (see the sources.list man page), you may find it easier to use an interactive tool:

# apt-setup

This tool walks you through the process of finding places to get Debian packages, such as CDROMs, Web sites, and ftp sites. When you're done, it writes out changes to your /etc/apt/sources.list file, so that apt-get can find packages when you ask for them.