No 3D prints without 3D modeling. If you want something 3D printet, and you are not satisfied with the 3D models available on the internet, you must get started with 3D modeling. Here are three possibilities for software that you can use to make your own 3D models.
The easy one: TinkerCAD (free)
An easy way of getting started with 3D modeling. The browser based app lets users construct simple 3D models that can then 3D printed.
The advanced one: Blender 3D (free)
Blender 3D, which surprisingly is free, is a far more advanced piece of software that also offers far more freedom. It is, however, a professional piece of software where user friendliness is not the focus. On the other hand, it is not limited to 3D modeling and 3D printing; if you want to render and animate your 3D models, that is also possible.
The complete one: Netfabb (paid)
The professional software Netfabb is made by the same company as TinkerCAD: Autodesk. Other than that it does not have a lot in common with TinkerCAD. The software has loads of tools for 3D modeling but also analysis and modifications of the models.