Do you need something that will identify and correct artist names and album names and track names as well? Or do you just need something that can find cover art?
What types of files are you working with? ALAC, FLAC, MP3, AAC?
Some tools work with some of the file types but not all.
What do you consider minimal quality for the album art? 200x200? 300x300? 500x500? Are you particular about image quality?
One tool you can try for album art is Album Art Downloader
. It is free. It searches for album art in several online album art sources (including buy.com and amazon and google). Album art quality is a mixed bag. It can load your Foobar library and cooperate with Foobar.J River Media Jukebox
can search J River's YADB database for artwork based on artist name and album name. It can also take a guess at artist, album, and track names based on audio fingerprints to tag files that are missing tags. The fingerprinting is very hit and miss. Depends on what's in their database and how well the fingerprints match. Jukebox is free. It will work with FLAC and MP3 and WMA. It will not work with ALAC or AAC.MusicBrainz Picard
is also an option. It can attempt to correct artist and album tags based on audio fingerprinting. It doesn't do album art. It is free.