Where can I find Literary criticism on Heart of Darkness?

I highly recommend shmoop.com for 'Heart of Darkness' in particular, because of its emphasis on the importance of context and its plethora of critiques and opinions, with commentaries of quotes organised under themes and lots of links to further literary discussion. Other good sites for literary criticism in genreal include 'revisionworld' and 's-cool' (type them into google) and books include Cliff's Notes, York Notes and the older Brodie's Notes. For a less mainstream approach, try searching under google Scholar, which will give you academic essays to look through, enabling you to pick out your own critiques.