An excellent resource for finding artists' signatures is through ArtistSignatureDictionary.com. A fan can type in the name of the artist he is looking for in the search bar in the center of the page and click Go.
Artist Signature Dictionary provides a photo of every signature searched for on the site, as well as the year it was signed. Both genuine and forged signatures appear on the site and can be searched for individually. Many listings also have a small biography for the artist linked to them, though the language of the biography is usually in the artist's native tongue.