Brass knuckles are designed to encase a person's knuckles in metal, making his punch much stronger. They're potentially lethal. Different jurisdictions regulate them differently. In some states, brass knuckles are illegal; in others, they may be legal to own, but illegal when carried and concealed.
Research the law in your jurisdiction. There are states that allow brass knuckles to be sold for "entertainment" purposes, but if you can't legally own them, don't purchase them. To bypass laws against them, some businesses sell brass knuckles as belt buckles or paperweights; others sell knuckles made of hardened plastic. Always err on the side of caution when purchasing to avoid criminal charges.
Obtain a permit. If you can legally hold brass knuckles in your state, you should apply for a concealed-weapons permit. Follow the specific process in your jurisdiction for obtaining one. A prior criminal record will prohibit you from getting a permit in most jurisdictions.
Purchase brass knuckles. They can be found at local weapons' stores. You can also purchase them online.