Making a 3D file can be a real challenge, but you have several options available if you are looking for an STL file for 3D printing. You can design it yourself by using a 3D CAD software, and then, you have to make sure that the file is 3D printable, check out the most common 3D printing problem first.
If you have no experience in design and still want an STL file to 3D print it, you also purchase a 3D model on a marketplace!