There are thousands of search firms in the world. The biggest firms in the industry are:
Kennedy Career Services (a division of Kennedy Information, a Bloomberg BNA business) publishes the annual Directory of Executive and Professional Recruiters (print edition $69.95, online edition $59.95); also called the recruiterredbook.com, this directory is a comprehensive list with searchable indexes of executive recruiters. Kennedy Career Services also maintains the selectrecruiters.com site, a fee-based subscription service that allows the user to search and connect with executive recruiters.
The publication Workforce Management compiled a list in 2006 of the twenty largest retained executive search practices in the world (by revenue), click here for that ranking list.