Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I find them via amazon or another music site - then highlight the album in iTunes and just drag the image into the box in the bottom left corner of iTunes.
 
don't forget you can also change if the artwork is shown for what's playing or what's selected... it took me a while to figure this out...
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.