You don't say when you are walking. Remember that September and May are extremely busy for all accommodations in this area, especially on weekends. It's a popular weekend getaway area most of the year, but it's especially crowded in those months.
The Abbey of St Louis du Temple in Vauhallan (just past Igny) has very nice pilgrim accommodation, but they are often all booked up, just as is St Thomas in Epernon.
http://www.abbaye-limon-vauhallan.com
When we walked last fall, we stayed in Vauhallan at the abbey, in Chevreuse at the Residence Hotel Ducs de Chevreuse (weekend, no other option anywhere, but not a first choice, for sure), Rambouillet in the Ibis Budget out by the N10 (ditto), the Epi in Epernon (St Thomas was booked up two months in advance).
However, for accommodations once you cross into Eure et Loir (Epernon-ish), contact the local pilgrims' association. They are super nice folks and will find you a private home if there's nothing else available.
http://www.compostelle28.org
It's possible that compostelle2000.org in Paris might have some other suggestions for Chevreuse to Rambouillet.
http://www.compostelle2000.org
It's a very pleasant walk. For the section in Eure et Loir, please check out the pdfs from compostelle28. Excellent clear directions.
ETA We did Epernon--Chartres in one day, but the
amis can find you a place to stay in between if you don't want to do such a long day, or there is an inn in Jouy:
http://www.lauberge-de-la-gare.sitew.com/#Accueil.A
The site is about the restaurant, but they do have a few rooms, too.