Google Image search offers an option to search by license, and you can search Flickr by Creative Commons license.
Remember, it's not just images that have copyrights on them, but also fonts! FontSquirrelhas a good selection of free fonts and you can find some at dafont, too, but be sure to use the more search items to ensure your search only returns free fonts. Watch for kerning issues.
Last but not least -- just because you can design your own cover, doesn't mean you should! Spend some time at LousyBookCovers to see what not to do! Try finding a cover designer on Fiverr (be careful) or check out Thrifty Book Covers or see my work and make an offer.