Where else can I get this?
On the World Wide Web!
iTunes will find the artwork if the album is available in the iTunes Store and your metadata precisely matches the Store data, in every respect (spaces, punctuation, spelling).
If iTunes does not get the art, simply use a web search to find the correct image.
To paste an image into a song file, highlight the file, use the command File > Get Info, open the Artwork tab, and drag the image into the space for art (or use Copy/Paste).