Oddens' Bookmarks is including an increasing number of single maps from early atlases and series [this page is concerned with sites only] so it is well worth checking there if you are looking for a specific early map [though note that this site is no longer being maintained]. For further details about quality see About the listing of early map image sites; and for map images issued in CD-ROM form, as slide sets, etc., see Boudewijn Meijer's MapRef page