Páginas donde puedes descargar mockups gratis:

https://graphicburger.com/mock-ups/ 
https://www.freepik.com/free-psd/mockup 
https://www.pixeden.com/free-graphics/ 
https://www.mockplus.com/ 
https://placeit.net/ 
http://www.pixeden.com/ 
https://www.mockupworld.co/all-mockups/