There are loads of great online sites- try Gum tree
http://www.gumtree.com/london/2548_1.html
Also easyroommate.com, flatemateclick, roombuddies
I would also advertise in the paper in whatever Borough you would like to live, if there is one.
You can go through estate agents but be prepared for them to reference you to death, take 2 months rent in advance as well as a months deposit, and possibly a guarantor. They will also be more expensive as they take their 10% commission out of the rental price.