The following list is not exhaustive, but represents a collection of resources that we have used with success in the past. Feel free to suggest additional resources if your favorite hasn't made the list!
Thingiverse "is a thriving design community for discovering, making, and sharing 3D printable things. As the world's largest 3D printing community, we believe that everyone should be encouraged to create and remix 3D things, no matter their technical expertise or previous experience."
Cults "is a digital marketplace for 3D printing and design...You don’t know how to make a 3D file but you want to 3D print objects? Cults is for you! Browse our different categories and download the best 3D printer models for free, open-source or paid mode."