There are tons. Some are more exclusive like
www.tiffany.com or
www.baileybanksandbiddle.com, and others are more mundane (like
www.zales.com).
One of the best online sources if bluenile.com, although Amazon is now very price competitive in selling jewelry. It depends on what you're trying to find or to learn, but search Yahoo or Google with the keywords you want - there are tremendous resources out there!