Check IMDB, the Internet Movie Database. This website lists films and television series that have been produced since the creation of the medium. It will list the distributor of the film in the notes. Check the distributor's website to gather information about sales of the movie and where a copy can be purchased. It should list the format in which the film is available.
Check the most recent addition of Bowker's Complete Video Diary, which lists 73,000 titles and details information about the distributors. Check the distributor's website or call to find a copy of the film for purchase.
Look on Amazon to see whether it carries the rare movie you seek. Amazon has a large array of old movies at reduced prices, including some that might not be available at video stores.
Contact companies that specialize in finding rare films or classic movies that are out of print. These include A Million and One World-Wide Videos. Search the Internet for a list of these organizations.
Some companies specialize in providing out-of-print movies that have never been released on DVD or VHS, with non-commercial rights. They charge for the DVD and transfer costs but not specifically for the sale of film. These items are for collectors only and not for commercial use.
Relay as much information as possible about the movie to the video detective. Details such as director, producer, actors, year of production are important.
Be prepared to provide a credit card payment when using these online services.