It's likely that you know your article's field very well, or you probably wouldn't have got the assignment. Therefore, it's time to put that knowledge to work. Poke around your networks to find out who an accepted authority in the matter is. Speak with committees, elected positions in the field, professional organizations or alumni associations.
Or, you may want to think more socially, especially for lifestyle pieces. Your online friends or your local community may be good choices here. Find out who is buzzing about whom. When you say "your article's subject" who do people name?
Of course, hitting Google is a viable option, as long as you have the ability to determine appropriate sources of information.
Last, if all else fails, ask your editor for direction. They may have someone in mind that they'd love to see on their pages.
