Look at the photo credits. Most photos will give credit to the individuals involved with the project. This might include the names of the photographer, model, stylist and their agencies.
Contact the agency. You can search for the name of particular agency on-line. If the agency has a website, look in the contact section for its address, email or phone numbers. Once you know how to contact the agency, send an email or call it to introduce yourself. Then ask about the stylist you are interested in.
Look for the stylist's name on a stylist resource guide. This is an on-line guide with listings of stylists from all over the world. The guide will often include the stylist's portfolio and contact information. If the stylist you are interested in is listed with a resource guide, you may be able to contact him directly. Websites such as Onetalentsource.com are good places to start.