The following websites host models that others have made. Download them and modify for your needs. A huge time-saver if you can find a model close to what you need!
Thingiverse (http://www.thingiverse.com/) is the largest collection of files for 3D printing online
SketchUp 3D Warehouse (https://3dwarehouse.sketchup.com/) is a vast collection of SketchUp models, most of which are not printable without making them solid
GrabCAD (https://grabcad.com/) are engineering grade models, most of which will need conversion to allow import to SketchUp