Some of you may know about this already, but my nephew found this website which sells copies of public domain films.
www.retrofilm.com (http://www.retrofilm.com)
They don't sell to the average person. They only sell to businesses and such like TV stations. Also, they're extremely expensive. I once contacted them just for the heck of it and found out one of their prints runs in the hundreds of dollars.
Yeah, it's better to find "alternate" sources, but the site itself is interesting, as it does give a useful sense of what is actually Public Domain out there -- for your festivals or monster-movie shows!!
peter johnson/denny crane
http://www.archive.org/movies/collection.php?collection=feature_films
for all your public domain needs... free!!!