Apple’s Developer Marketing Resources offer great, high quality “Product Images”/ mockups for all their devices, free to use when designers and developers want to market their work.
Mockup Hunt is a huge collection of the best free mockups hand-curated from the most trusted mockup websites. Their collection is updated weekly.
Freepik Mockups is a platform where designers could find 100% free PSD mockups and Photoshop resources of the highest quality. And more…
Mockup`s Design is a simple site where designers can find premium-quality free mockups, to use in their projects.