There is a site called Turbo Squid. It has tons of models both paid for and free. Due to many scammers out there and digital thieves I would not purchase and models from there but would use the free ones as placeholders till you have your own made or purchased from a more viable source. Below is the link to the site just adjust the search filters to show only free models.
http://www.turbosquid.com/Search/?max_price=0&keyword=free&min_price=0