well, I did the same research (it's actually near to be totally done...) and I can tel you that one of the best source you can find about Sarah Kane is the book of Graham Saunder "love me or kill me..." which is really complete and may help you to find other sources. You can also go on wikipedia, in English, and look into their bibliography; they've got good articles and a complete pack of sources.
also, one of the most interesting tings I've found, is this;
http://www.rhul.ac.uk/drama/staff/rebellato_dan/index.html. It is a really good interview, and you have the audio (... that was on tape... so the quality is sometime... bad) and the Pdf to understand everything.
good luck in your research
-Gabrielle