This may not be exactly what you are looking for, but I use CoverScout by Equinux, and it has a built in search feature to find the art you want and then it adds them to itunes.
Clutter is what I've used for years. Once it finds the cover art, it has a "Copy Cover Art to iTunes" menu item (or something like that).
Of course, iTunes 7 has that feature where it'll download missing cover art from the iTunes Store, but that only works if the store has the song and yours is named the same way. Another drawback to the built-in iTunes feature is that apparently cover art obtained this way is not stored in the music files themselves. Yes, it saves space by only storing one copy per album, but then the artwork is lost if you move the files elsewhere.