It is possible to view Mexican movies for free on YouTube. Search for the movie you have in mind to see if it is available on the website. Also, the "Spanish Movies" YouTube channel has a variety of free movies and documentaries, some of which are Mexican.
While some movies are free to watch on YouTube, other, generally newer, films carry a rental fee. Rental movies are only available to view for a short period of time, and prices vary depending on the film. It is also possible to buy Mexican films from YouTube for the price of a regular DVD.