I have used one which gave you a 30 day free trial and about 12 instant checks (you can buy more)
It was good but if you are after finding EXACTLY where she is it is next to useless, it will give an aproximate location to within maybe 600-1000 yards but thats about it, so it will tell you she is in say one part of town as apposed tpo another or manchester as apposed to liverpool! but you could never use it to actually find her?
That is my own experiance of it but it might vary from area to area. (manchester by the way)
w w w.childlocate.co.uk
Mike