I rip mine from DVD's, I have several hundred and buy more all the time. Anymore when a new one comes out, I buy it, burn it to a blank and rip it to mp4 at the same time. That way I get the aspect ratio right and everyone isn't stretched. You can use Nero 7 or above for this.
It converts them to MPEG-4. I also use Plato video converter to change the avis and wmvs I have to mp3. It's pretty awesome and will let you convert to 3gp, or format for PSP, zune or a couple others.