You can download tv shows from iTunes, or you can rip your favorite dvd's and put them on your Video iPod. I've done it myself, it's pretty easy to do. Just do a search for ipod video encoding on google.com.
I did a google search and found a couple of sites that give you unlimited access for a monthly fee. I did not do a search on the sites to see what kind of selection they have. My wife looked at "Vongo.com" and found a fair number of movies, however the format was only compatible with one of iPod's competitors. You can probably download another format and then convert is a similar fashion to the previous posts. I have not been in a rush to do this myself.