Discogs
Large selection.
Sort by genre, style, and format.
Sign up quickly using Facebook or Google.

bing
Somewhat cluttered album pages.
Discogs is one of the largest online databases for audio.
Musicbrainz
Large mix of search options.

Download the entire database for free.
Confusing site to navigate.
Musicbrainz offers a huge catalog of music information with included artwork.

Internet Archive
Free.
Not a dedicated art site.
The Internet Archive offers many gems, both old and new, including album covers.

Album Art Exchange
Free service.
License model for the art is legally questionable.
Amazon and eBay
Established marketplaces.

Hit-or-miss based on current product availability.
No guarantee that covers are available.
If a cover is available, it may not be good quality for album art.

bing
Web Searches
bing
Easy to useit’s a search like any other.
Google and Bing offer image thumbnails.
Limited to content that’s available on the open web.
For rare content, you might get lucky by searching your favorite search engine.