I've searched around and can't find the packages for Lenny (5.0.10) anymore. You may have to download the ISO for the distribution and get the packages from that. The ISO files can be found here:
http://www.debian.org/releases/lenny/debian-installer/
I've attached the copy of find from that distribution here for you, to save you some trouble. There are many other utility programs that will also work, and you should get those from the Debian distribution.