I've never had to find an apartment before, let alone in a foreign country. What am I doing? If I don't get scammed I'll consider myself lucky.
Anyway, the assistant's handbook has a bunch of housing websites on page 16, but the links don't work so I've typed them up here, for easy reference:
Housing websites
http://geo.craigslist.org/iso/fr
www.pap.fr
www.seloger.fr
www.immostreet.fr
Finding a (French) roommate
www.kel-koloc.com
www.appartager.com (recommended by the other assistants in Nice)
www.colocation.fr
Housing on a limited budget
www.mapiaule.com
www.location-etudiant.fr
www.locservice.fr/logement-etudiant.html
www.annonceetudiant.com
www.capcampus.com/logement-1159/
Housing for students
www.cnous.fr
www.adele.org
www.estudines.fr
www.icade-immobilier.com (updated from handbook)
www.logetudes.com
You can also look through your local Centre Regional d'Information Jeunesse by searching CRIJ and the name of your region.