Torrent sites is the standard method (isohunt, thepiratebay, demonoid, etc).
If you do not like the hassle of setting up a torrent client, you can also watch them directly on the net, like on surfthechannel.com
Try this site. You can either download or watch online.
According to reviews they have the largest selection of all these types of sites. And there are both paid and FREE membership options. Virus and malware free also (according to cnet)