Hi there,
Sorry this has taken me a while to reply to. You're probably done with your essay by now!
My suggestion, if you have easy online access, is to type in "+Coleridge +"Kubla Khan" +criticism" into the Google search engine at www.google.com. The first three good starts that I found were these:
http://www.cs.rice.edu/~ssiyer/minstrels/poems/30.html
http://www.eiu.edu/~multilit/studyabroad/lyricalballads/comp3d.htm
http://users.ox.ac.uk/~scat0385/eclipsed.html
Also, with this poem it's good to focus on some specific aspect of it, like imagery or its questionable origins. If you need help, please feel free to e-mail me at lisaml1203@yahoo.com.
I hope this helps.
--Lisa