The nice thing about finding an apartment online is that many times you can by-pass the agency. That way you avoide paying an agency fee. Here are some links to listings online in the Poitiers area. For students coming to ESCEM - look for an apartment in the centre-ville. That will put you close to the school.
Also, it is best to rent a furnished apartment. As a student you will not be staying for a long period of time, and renting a furnished apartment will allow you the freedome to go home without worrying about furniture (weather it be selling it, or shipping it home).
Another choice is to go through an agency. While you do encur agency fees, if you make an appointment with Foncia, they have an agreement with ESCEM, where you get a discount of 20% off the charges. Also, you are able to tell them exactly what kind of apartment (size, furnished, electricity/hotwater included...etc...) that you are looking for. They will find an apartment to match your needs, and also one in your price range. They will take you to visit the apartments they have found that match your criteria, and then you are able to make a choice. Also, if there are ever any problems with your apartment (such as small repairs to plumbing, etc) rather than trying to track down a landlord, you go right to your agency and they deal with the landlord.