Mockup files

https://www.mockupworld.co/

https://graphicburger.com/

https://elements.envato.com/graphic-templates

Creating Adobe Photoshop Templates for Mockups (Full Process!)

https://www.youtube.com/watch?v=ZSEmthIpWi0
How to Design Mockup in Photoshop | Adobe Photoshop Tutorial

https://www.youtube.com/watch?v=f5FRcbfHKck
 
How to Make Perspective Mockups in Photoshop

https://www.youtube.com/watch?v=eeNJr3YBw6Q&t=193s

Or use Adobe Dimensions