Use Netflix as the best source for Arabic films that have been produced to the standards of Western DVD releases and accurately subtitled. The Internet-only package was $7.99 per month, as of January 2011.
Find movies of less quality that have been uploaded onto YouTube or Google video. Searching using the Arabic alphabet is the best way to find these movies, however the films you find will probably not be subtitled. Films that have not been widely distributed in the West will often be available in this way.
Go to the free Sotwesoora website for Arabic TV shows and films. This website features a huge selection of Arabic films and includes a search function that allows you to input the title either in Arabic or transliterated into the Latin alphabet if you know the commonly used transliteration of the title of the film you want. Hardly any of the films available on Sotwesoora will be subtitled, but this website may be the best online source for films only released in the Arab world.