TinkerCad is a web-based 3D modeling program. This free to use program offers tutorials that will help you learn the program and be able to create your very own 3D models.
Thingiverse is a online repository of 3D models that you can download and print.
The NIH 3D print Exchange contains various 3d models of biological items. These include molecules, anatomical features, cells, and even lab supplies.
Yeggi is a 3D model search engine that links you to the primary repository for the files.
MyMiniFactory is a 3D model database with a large selection of models sorted into categories, including educational