Generally when designing a piece of industrial hardware or a product prototype designers have to create all the models from scratch. But there are online resources full of functional 3D models that are explicitly designed for functional use. There are also resources for custom on-demand 3D Printed parts.
McMaster supplies 3D Models for many of their hardware products. These models are provided in the .STEP format so they can me edited and integrated large CAD assemblies, or printed to use in prototyping.
Part Mason is a partner company to Slant 3D. It is a repository of 3D printable models that can be created and used in as final parts in applications. Parts can be downloaded in a number of formats from 3D Printable .STL's to editable .STEP. They can also be ordered printed at any volume, whether it be 10 or 1000. Part Mason is basically a fully digital on-demand parts store.
Finding the 3D models is a challenge by itself. But then you need a 3D Printer that can reliably print those parts. Check out the Mason 3D Printer as a resource for 3D printing functional industrial parts.