Every once in a while, someone asks about finding an apartment in Paris.
Here are two on-line services
lokim.com. Used to be called paris-be-a-part-of-it. Luckily they changed their name. Reputed to be "user-friendly and professional, and there are a range of prices."
France-USA Contacts has a very active site with professional and private rental availabilities: fusac.com.
Any other suggestions?