first of all, have you got a topic in mind? Once you have an idea about the area, try to find some articles, papers etc written in that area. Look at who is writing them and find one or two whose style you like. Read several of their articles, even if they are in slightly different areas from the one you want to research, then e-mail them, comment favourably on one or two of their articles and tell them about the area you would like to study, how you think that their work would inform your own area, and how much you would like to study with them as your supervisor. You have nothing to lose and everything to gain by using this sort of approach, it shows you can research, you know what area they are working in and how your work would fit in with it, and if they can't help they may know someone who can!