Best Mates is a Rare Fortnite Emote. Best Mates was available via the Battle Pass during Season 3 and could be unlocked at Tier 63.
Best Mates was made a Traversal Emote in the Fortnite 11.40 Patch.
How-to Get the Best Mates Emote
Best Mates is part of Season 3. If that season is still currently in the game, you can obtain this item by purchasing and/or leveling up your Battle Pass.