It was first introduced during Chapter 1, Season 8 (March 2019).
Purchasing this skin for 1,500 V-Bucks includes the Rainbow Clover. Fortnite Rainbow Clover Backpack
The Rainbow Clover is not sold as a standalone item. Instead, it is bundled with specific outfits in the Fortnite Item Shop : It was first introduced during Chapter 1, Season
Part of the Green Clover Set , which includes other festive items like the Sgt. Green Clover skin and the Pot of Gold harvesting tool. Fortnite Rainbow Clover Backpack