I don't mean to sound cliche but a lot of information on self harm is centred around young people, you may find it hard to get supporting evidence for your essay.
Information fact sheet on understanding self harm:
http://www.mind.org.uk/Information/B...+self-harm.htm
youth and self-harm:
http://www.samaritans.org/your_emoti...self-harm.aspx
Again young people:
http://www.mentalhealth.org.uk/publi...EntryId5=38714
Self harm/self help:
http://www.mentalhealth.org.uk/publi...EntryId5=43586
A list of organisations that help support self-harmers:
http://www.nshn.co.uk/resources.html
Also try the World Health Organisation, the national office for statistics, erm not sure of what else my favourites page has craaaaaaassssshhhhhhheeeeeeeeeeedddddddd, hope those aid a little, I havent had time to read through them all x