Thingiverse (http://www.thingiverse.com/), owned by 3D printer manufacturer Makerbot, is by far the most popular 3D printing model site. Most models are free, but even on free models the uploader may choose to set restrictions on usage, for example, not for commercial use.

Most of the models are in .stl format, ready to download and print. Models can be uploaded in nearly any format, including .skp.

The 3D Warehouse (http://3dwarehouse.sketchup.com) is SketchUp's very own repository of models. It's astonishingly extensive—everything from Star Wars X-Wing models and people in various uniforms and poses, to architectural models and useful household objects, and nearly everything in between can be found in the 3D Warehouse.

All models are in .skp or another SketchUp compatible format, meaning you can open them in SketchUp without converting the file format. Models can be downloaded using your web browser or directly into your SketchUp model using the Components browser.

Most models in the 3D Warehouse were created for use in visualizations, so they're not built with 3D printing in mind. You will either have to fix them to be printable, or rebuild them to be printable. Rebuilding the model is often the quicker choice. Even if you need to rebuild a model, having a model to start from and use for reference geometry can be a time saver. Rebuilding a model using an existing model as reference is covered in Chapter 8, Modeling Architecture for 3D Printing.

GrabCAD (https://grabcad.com/) is popular with professional engineers. Most models are high quality, mechanical type items. Relatively few are in a format compatible with SketchUp, but if you message the model owner and ask nicely, they may convert the model to a format like .stl that you can import into SketchUp.