Here are some free content that you might want to consider for your mock up designs or other designs works that you do. Consider them in the following links.
Picjumbo has an immerse amount of high quality pictures that you can consider.
All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty-free for anything you like, even in commercial applications. You don’t even need to do any attributions 🙂
Six kind of badges and vector artwork that you can consider.
dafont has a huge collection font that you can download and use. Choose from the range of categories that they provide. It even includes a Preview text to see how your text look like.