Here is an excellent SEAS page containing details for incoming students: http://www.seas.harvard.edu/academics/graduate/information-for-new-graduate-students
These GSAS websites have valuable information for new students
To extend options for students to find housing outside of Harvard on- and off-campus housing, here are some Facebook pages that list options for apartments or houses in the Harvard area! You can post that you are searching for a place or look at posts from currently available places.
https://www.facebook.com/groups/HarvardMITHousing
https://www.facebook.com/groups/1627481087579159
You will have to request access, but mentioning you are a Harvard student seems to work well!
