Since Hitman Sniper Challenge was never sold as a standalone retail product, it cannot be purchased on Steam, GOG, or the Epic Games Store anymore. The only way to legally obtain it today is by finding an unused pre-order key for Hitman: Absolution —which is nearly impossible.
Try the free Hitman Sniper mobile game or subscribe to to play Hitman: World of Assassination . The sniper maps there are superior, legal, and safe. hitman sniper challenge ocean of games