Most departmental websites have listings of their faculty and descriptions of their research. Read through that and pick someone who you want to work with. Contact that person directly. Start your email by introducing yourself, state your interest in the specialty, and say something about why you are interested in working with them specifically. Ask if they are willing to take a student and ask for a meeting. If you have previous experience, say something about that as well so that they know they won't have to train someone from scratch. Most faculty welcome these types of inquiries.
Going through the department chairman may work. However, they are usually very busy and may not take time to discuss things with a student that they can easily look up themselves.