the largest archive of original dvd covers is available here
http://www.cdcovers.cc/covers.php I use the original covers for my dvd copies except edit the spines to half width since use the thin cases to get twice as many movies in the same shelf space. These have enough resolution for perfect 1:1 prints.
For mp3 album art this free program will find them automatically if the id3 tags are correctly filled and one click embeds the art in the mp3 http://kempele.fi/~skyostil/projects/albumart/
otherwise just put in your keyword and it will find the album cover.