copyright free, and they usually are not, downloading them is not altogether legal.

A lot of clips are produced using Flash and as such do not easily transfer to iMovie. They have to be converted from Flash to something that iMovie can accept.

How about supplying an example of the type of movie clip that you want to download and put into iMovie.

As an aside, it is never a good idea to post your email address in a public forum. I will ask for it to be removed.