All the repositories for old Debian versions can be found in the archives.debian.org repository:
deb http://archive.debian.org/debian/ lenny main non-free contrib
deb http://archive.debian.org/debian-security/ lenny/updates main non-free contrib
deb http://archive.debian.org/debian-volatile/ lenny/volatile main non-free contrib
deb-src http://archive.debian.org/debian/ lenny main non-free contrib
deb-src http://archive.debian.org/debian-security/ lenny/updates main non-free contrib
deb-src http://archive.debian.org/debian-volatile/ lenny/volatile main non-free contrib