You are talking about movies that are now in the public domain. There are a few sites where you can get them.
http://www.archive.org/details/feature_films
The above link is the the public domain films on the Internet Archive.
http://www.publicdomaintorrents.com/
The above link has lots of movies that are in the public domain, that you can download with a bittorrent client. Also, they are in a number of formats.
There are others too, but these should get you started... ;)