Itunes. But I'm assuming you want free movies and I'll also assume you did a google search and found nothing. I don't really know of any site that offer mp4 movies so your best bet would be to convert your own movies.
i know where... come check on my threads hehehe there should be one saying uhh.. movie thread in my sig it should help.. there are some movies that are mp4.. you can convert the video yourself if its in another format.. winavi program will be on the anime thread and movie thread of mine.
Torrents.. Then the awfully painstaking process of converting to MP4. It's a shame that iPod's don't support many video formats