Irty Pretty Things mixes realism and symbolism in a highly effective way. Discuss

Dirty Pretty Things 2002 UK directed by Stephen Frears

You can often get good material from general books. You dont have to read all of a book or article: ashort passage can be used as a singboard for your own thinking and arguing
If you cant find something on the specyfic book or movie you are writing about, you can adapt ideas/ approaches which have been applied to other texts and apply them to yours. You can also use the texts/films you are not writing about as useful comparison, cross-references etc to develop a point or argument.
context/background/approach/theories can all be helpful ways of beefing up an essay or argument if you lack criticism directly on the text/moviebut use the context material on the way through, not ina lump at the begining

the web has lots of material
Benyahia, Sarah Casey 2005 Teaching Cntemporary British Cinema, London: BFI -PN1993.8 G7 B4case studies on Last Resort, Dirty Pretty Things, My Son the Fanatic

Any other sources those are available