There used to be away to get iTunes artwork manually but Apple shut it down, which makes me mad as there is some artwork missing for my songs that I KNOW Apple has but it just won't apply it to my songs.
For me it was easier to use a combination of wikipedia and amazon...I was able to get some rare album covers from only wiki for The Beatles that was pretty cool...
I get mine off of Amazon (the art they provide be default is of such a low resolution that I go right to the customer supplied images). If there isn't any customer art available, I'll go to Google Images. Sometimes I'll go through the artwork that I have and compare it with the art on Amazon. If what I have is better (or the album only has Amazon's cover) I'll upload the better one. I like to think of it as a bit of a club, one where we have to look to Amazon after Apple closed the iTunes art grab loophole. Its up to those who have good images (or rare covers) to upload them to Amazon for the benefit of all. Viva! la revolucion!