I bought a Dissertation book by Palgrave. It's pretty helpful for writing your dissertation, but not so much on the 'finding a topic' front.
I am doing my Dissertation on Media and Entertainment Law - As this is the area I want to go in to. I won't study this on my degree, so it's something completely new to me, which is of great interest to me too. I then chose Privacy Laws between two different countries, and how they differ, what would happen is the Laws from one were implemented in the other and so on.
You could pick an area of Law you've already studied, where it may have made you think about things, changes that could be made, or how that system works compared to a system in another country or something. Or an area of Law you want to go in to. Speak to your Dissertation Supervisor - they'll give you some basic ideas or examples.
As the other poster said, pic something you're interested in - you'll be living with this topic for a looong time!