I make a lot of covers and use this site most often because they have a ton of diversity. It's not always easy to find what you're looking for on some of these sites, but their search parameters help and the availability of stock is better in many ways than some of the other stock sites.
Tip for consumers: The photos usually have a ton of tags to help you search for them. You can get really specific about what you want and find what you need fairly easily. Also, make sure you put things in a favorites / projects folder because you will definitely want to come back and get more later!
Products used: I use mostly photos although there are vectors and videos also available.