It’s especially important that none of your Zooppa ads contain any copyrighted material. Below are resources for inexpensive/free content. However, it’s important to note that just because content is free, doesn’t make it legal to use in a winning Zooppa submission.
In order to use a piece of music, an image, or video footage, you must have proper legal permission to use the content in a commercial venture. Please check your license terms very carefully. After all, if you submit, you are in fact transferring copyright of your content to the client.