You can search for pre-existing models on the Internet to print as they are, or to modify using 3D modeling software. If you would like to print using a pre-existing model, make sure it is saved as a .STL file when submitted. 

Websites where you can access pre-existing models

• Thingiverse Makerbot's searchable design library community.
• 3D Warehouse  Sketchup's searchable design library . 
• Instructables has a 123D community