The good news is that renting a flat in Cergy is much less expensive than in the center of Paris. However, you should start looking for accommodation as soon as you know that you are admitted into the program. The best flats are rented very quickly.
You should know in advance that many papers can be asked (especially to prove that you will not have trouble renting a flat).
Other general websites :
http://www.pap.fr/ : this website is (mainly) for owners that do not wish to rent their flat through an agency. An advantage is that you avoid paying agency fees (equivalent to one month rent of the flat you move into)
Websites to find a flat to share with other people :
A website with owners who rent a room in their house :