If you don't want to create your own 3D design, there are online sources for 3D models that other users have created and submitted for public or professional use. Sometimes the part you may need for your project has already been designed and is available for free from one of these sites:
These 3D model sites offer both free models and those for sale. Costs can range from a few dollars to a few hundred dollars.
Brigham Young University Science & Map Department. (2017). 3D printing guide. Retrieved from http://guides.lib.byu.edu/c.php?g=216600&p=1429612
Gonzalez, S. R., & Bennett, D. B. (2016). 3D printing: A practical guide for librarians. London: Rowman & Littlefield.