Instructions
Search at a local retailer that specializes in "classic" video games systems. Many large cities have at least one independently owned store that specializes in carrying games and systems that have been discontinued. These stores will occasionally carry third party peripherals, such as a "GameShark" for the Game Boy Color.
Purchase a "GameShark" from an online retailer. Amazon and Google Shop often act as a hub for multiple retailers, ensuring a wide selection of inventory and safe purchases. These retailers often have links to purchase old accessories, such as the "GameShark" for the GBC.
Purchase the device from a seller on eBay. The eBay site allows users to create their own auctions, so there are commonly useful or classic items being auctioned for a discounted price on the site. Check the site frequently to keep up to date when a compatible "GameShark" is being auctioned.