I know that Upenn has a housing listing at this website,

http://www.business-services.upenn.edu/offcampusliving/

there is also listing on the citypaper, under classifieds section

http://www.philadelphiaweekly.com/



be prepared that you will need a temporary place once you get here,
then spend 2 weeks or longer to check out apartments before you can
sign the lease and move in.