Here are some placement agencies that specialize in overseas positions:
Search Associates
Search works with candidates who are and are not certified to teach.
Major Fair: Cambridge, Massachusetts (February)
International Schools Services
Many schools ISS works with require candidates who are certified to teach.
Several fairs each spring, beginning in February.
UNI Overseas Placement Services
UNI works only with certified teachers.
Major Fair: Waterloo, Iowa (February)