You might also try the Search the Internet plugin. You'll still have to copy and paste from a website, but it makes it easy to look up an author and/or a book.
I use it so frequently to find covers that I almost never even use Calibre's built-in metadata downloader for covers, but you might prefer to use the metadata downloader first and Search the Internet only when that fails.
My favorite searches are FantasticFiction for author, Google Images for title, and (because I mainly read SF and Fantasy) ISFDB for author. Between those three, I've found decent quality covers for 99% of my books.