Chicago Extra Casting Agencies
This list of casting offices that find/hire background actors may not be accurate or up to date. I know at a minimum that Ambassador Talent Agents and Joan Philo Casting both did at one time do extras casting, though the contact details on the Illinois Film Office web site may not be correct. Caveat actor.
2011 update: Recently ExtraOrdinary Casting and Joan Philo Casting have been doing background casting on a lot of the TV and film shooting in Chicago. Please tell them you heard about them from David August, thanks.