I have found allmusic.com a good source for cd covers (and for other info about a release). If the cd cover is the same as the album, then that would be a place for lots of them (assuming a cd version exists).

Other sources you might want to look at (depending on the genre):

[url]http://www.lpdiscography.com/[/url]

[url]http://www.discogs.com/[/url]