Existing models you can print:
https://www.thingiverse.com/
CAD and Design Software
FreeCAD – https://www.freecadweb.org/
OpenSCAD – http://www.openscad.org/
Tinkercad – https://www.tinkercad.com/
Fusion 360 – https://www.autodesk.com/products/fusion-360/
3D Printing Services
https://www.3dhubs.com/
https://www.shapeways.com/
Slicing Software
https://www.simplify3d.com/
https://ultimaker.com/en/products/ultimaker-cura-software