The built in Amarok Cover Manager only allows fetching from amazon.com, which may be fine if your music collection largely consists of popular music. But if large parts of your music collection are vinyl rips or releases on underground labels, searching discogs.com will yield much better results.
This script allows fetching album cover art for the currently playing track from discogs.com by clicking onto any track in the playlist and selecting:
"Fetch covers from Discogs" -> "for currently playing track"
Already existing local album art (image files from the currently playing track's folder) as well as relevant album art from Discogs.com can then be browsed / saved / deleted.