Depending on how old a version you are looking for, my only guess would be to try all of the mirrors located at the following link: http://knopper.net/knoppix-mirrors/index-en.html You would have to go to each one and see what they still have available. It would require a little leg work, but I'm sure it wouldn't take too long. Some other distributions are based on older versions of Knoppix, like Knoppix For Kids, etc.

WaKa provided a solid link that has a few older versions. One of the mirror sites might go back even further. I have not looked in a while, but I'm sure you might find some older ones.

The main question would be why you might need an older version. They might be buggy and would not recognize as much hardware as current versions. If you are looking for something to run on an older machine, there might be a better choice with a different distro that requires less overhead, although the currrent versions of Knoppix will run on quite a bit of the older hardware. You might have to change to a different Desktop using a cheat code to get a little better performance on the older stuff.