If using 3D models created by others is not your way, have a look at the following tools allowing you to create your own models. Of course exports from these programs can be imported into the Solidator Studio.
Autodesk 123D is a great free CAD tool that can export stl files and Import STEP (stp) Files. Use this tool if you are just starting to learn modeling and GrabCAD.
Blender is targeted at creating stl Files directly. The models created this way, however, lose precision when scaled.