1. run iTunes;
2. create a new playlist and copy into it all your songs/albums without cover;
3. export this playlist as text (.txt);
4. point your favourite browser to http://art4itunes.com/getart/uploadtracks.php, upload your .txt there and wait some minutes.. you will get one or more webpages with all the covers you need ready to be downloaded e drag&dropped into iTunes!
The search for some albums, also the famous ones, it could give you some wrong covers. If it happens to you, just search it with other methods 🙂