You can search for pre-existing designs on the Internet and either print the model "as-is" or modify the model using 3D modeling software.
Tinkercad - a browser-based 3D design platform, now part of Autodesk (free version available)
Sketchup - comes in free or pro versions. Get the Sketchup STL from the Extension Warehouse
Blender - a free and open source 3D creation suite