I don't know if everybody is aware of this website or not...but I figured it would be useful to share just in case.
This website has a list of sites where you can obtain legally reusable media whether its sound effects, music, images or video. Some of them will already be familiar to you such as stock.xchnge, archive and freesound but there are some other great sites on the list aswell. And even better it also tells you how to better utilise search engines we already know like google to obtaining material you can legally use. Word of warning with the last option though as sometimes it comes up with images from flikr that may only be used with permission from the creator, but it's fairly easy to check the source and whether you have to get permission or not.