Some information, with a link to images of older debians, is right here
. Here's where the repositories for your sources.list are: http://archive.debian.org/debian/dists/
So, for Sarge (Debian-3.1), you'd put the following in your sources.list:
deb http://archive.debian.org/debian sarge main contrib nonfree
The images only seem to go back as far as Sarge, from what I see.