First, hang tangs are handy to have attached to a piece of jewelry you plan to sell because it offers a spot for you to include information about the materials used in the necklace, earrings, or bracelet. There are a number of ways you can get customized hang tags, and I have an article that gives you a few different options for this: Jewelry Hang Tags and Earring Cards.
If you don't mind buying off the rack so to speak, check with vendors such as Rio Grande. They have a catalog devoted to items such as jewelry displays and tags.
Finally, if you are looking for a tag that will permanently stay on your jewelry, your signature in a way, then there are also various options for creating metal tags to attach to your jewelry. See Signing Your Jewelry and More on Metal Stamps.