Impasto wrap is a Rare Fortnite Wrap. This gun wrap is currently unreleased. It's an upcoming gun skin that have been leaked but hasn't been put on sale in the Fortnite Item Shop yet! Gun wrap Impasto is reactive. It means it changes or evolves (reacts) depending on certain things in game (damage dealt to opponents, if it's day or night...)
The weapon skin Impasto belongs to Chapter 2 Season 5. This gun skin is a leaked weapon skin, an upcoming item that will release in the Fortnite Battle Royale game. Therefore, you cannot buy it yet at the Fortnite Shop or get it by any other method.