Illustrators are friendly and helpful but be considerate of their time. If you contact them individually keep your requests short and to the point and realize that they may not be able to answer right away. Check their personal website to see if any of your questions are answered there. Be polite, and make sure you thank them for any information or feedback that they give you. I have found illustrators in all fields to be very generous with advice and friendship.
Join professional organizations and attend their conferences. There may be opportunities to show your work to an art director, display your work, have a professional critique, and meet up with other illustrators, publishers and editors.