1. Mockups-Design | https://mockups-design.com/
Mockups-Design is an incredible website with hundreds of free mockups. You can get mockups of anything from apparel to packaging, devices, posters, billboards, and so much more. Mockups are downloaded in .psd format.

2. Pexels | https://www.pexels.com/
Pexelse is an online website to get free digital images and videos for anything you may need. Most images and videos are uploaded in high resolution and are completely free.

3. CC-0 Vectors | https://publicdomainvectors.org/
Public domain vectors is a great website with thousands of free vector images that you can use in your design work.

4. Pattern Monster | https://pattern.monster/
Pattern Monster is an online site that allows you to easily create patterns, backgrounds, and more for your design projects. They allow you to download them in .svg, .png, and more.

5. Gumroad | https://discover.gumroad.com/design
Gumroad is a great place not only to find purchasable design assets, but it's also an incredible place to sell them on. Check out Gumroads design category to browse their digital assets created by independent creators.