A good article by Michelle Joyce in Transitions Abroad containing a small list of schools in Mexico City.
Monica Gomez of Global ERS, a FREE, highly-efficient one-woman placement agency based out of Puebla but staffs throughout Mexico.
Teacher MexConnect, another staffing agency with a more comprehensive network in Mexico, charges a fee of $25 or $60 depending on the number of resumes you want to submit.
El Universal, a major DF newspaper, has an extensive online classified section. Click on 'aviso', then 'empleos' for job opportunities.
Also, the January 2009 publication of Chilango, a local magazine, provides a guide to private schools in Mexico City, divided up by location. Almost all are bilingual schools, and many will be hiring in the coming months.