FREE 3D MODELING SOFTWARE TO DESIGN FOR 3D PRINTING
DESIGN IT YOURSELF IF YOU CAN'T FIND 3D PRINTING FILES FOR FREE
If you want to design your own prototype object to make custom 3D printed parts or modify an existing design for 3D printing, you can use Tinkercad or 3D Printing Blender Toolbox add-on. Using these free & easy to learn 3D print design software, you can make any 3D printing files for free.
Thinkercad is a completely free 3D modeling software to design for 3D printing. Within minutes, you can easily make dimensionally accurate 3D printed gun models, 3D printed house models, 3D printed organs models, or even create files to 3D print useful things such as custom replacement parts or prototype objects.
Blender is also a free 3D modeling software. But it's a little harder to use. However, with the 3D Printing Blender Add-On, you can design any 3D print STL files you can imagine, including turning photos into 3D printed lithophane or sculpting highly detailed models for 3D printing just like Zbrush (except that Blender is free & much easier to use).
3. FUSION 360
Fusion 360 has a free personal license to CAD design extremely complicated dimensionally accurate multipart assemblies such as 3D printed metal prototype objects, 3D printed gun (cool in theory but illegal in practice), & even 3D printed car. This is the best 3D modeling software to learn for CAD inventor, CAD drafter, & CAD engineer.