Not everything has a reference source specific to the title, however, that does not mean that you won't find a useful one. You just have to take a step back. For example since there is no source for 28 Days Later you might try to find one on zombies or zombie films.
Just like Jakyll and Hyde academics have examined the film 28 Days Later through different lenses. Here is an example.
Just like with the reference source, you may need to take a step back when looking for journal articles on 28 Days Later. So broaden your search from the specific, 28 Days Later, to the general, zombie films. Below are some examples of the search results. Don't get frustrated. Searching for journals is often a lesson in patience but there are sources in the database.