" Movie review card" by Personal Creations is licensed under CC BY 2.0
Here are a few online resources that compile entertainment reviews.
These print indexes may also be helpful for finding reviews of films prior to 2003.
These databases will help you find primary sources about films (i.e. newspaper and magazine coverage of a specific film).
Image: Motion Picture News, Nov-Dec 1918, via Lantern (Media History Digital Library)
Digital historical newspaper coverage from 1880s to 2000s. Includes Atlanta Constitution, Baltimore Afro-American, Boston Globe, Chicago Defender, Chinese Newspapers Collection, Globe & Mail, The Guardian and The Observer, Hindustan Times, Korea Times, LA Sentinel, LA Times, Le Monde, New York Amsterdam News, New York Times, Pittsburgh Courier, South China Morning Post, Spokesman Review, Times of India, Vancouver Sun, Wall Street Journal, and Washington Post.