1) Mockup World
Mockup World is one of the best places to find premium and free PSD mock-up templates, it provides quality free web resources and graphic design templates.
Behance is the place where designers showcase and discover the latest works from top online portfolios. It is a great place to find quality free mock-ups for web resources and graphic design templates.
Graphic Burger brings tasty design resources made with care for each pixel. Free for both personal and commercial use. Have a bite!
This is an interesting platform to find many templates and mock-ups. The originality of this platform is that all mock-ups are created directly by the owners.
Free as part of Adobe CC. Create product mockups, brand visualizations, packaging designs, and more with easy 3D tools, all in the same app.