Heres how I found a place for you to get it.
First I went to a Knoppix mirror http://debian.tu-bs.de/knoppix_cd/. I noticed they had a directory called md5-old which contains all the old MD5 checksum files http://debian.tu-bs.de/knoppix_cd/md5-old/. Those MD5 files contain the name of the Knoppix ISO in its filename, for example: http://debian.tu-bs.de/knoppix_cd/md...-16-EN.iso.md5. So I copied the name of the ISO file KNOPPIX_V3.3-2004-02-16-EN.iso and I dumped that right into google http://www.google.com/search?q=KNOPP...4-02-16-EN.iso. The first item in google's list was the one I choose and it does in fact contain a ISO image of Knoppix 3.3 2004-02-16.
Enjoy.
http://ftp.surfnet.nl/knoppix/

-edit-
I downloaded KNOPPIX_V3.3-2004-02-16-EN.iso from that link & verified the MD5 from a different server and the ISO file is legit.
Enjoy.