There have been for P3's but DJI bought out a firmware update to stop them from working. Don't expect to see any for the Mavic. I wouldn't buy them anyway if you could. Mavic batteries are cheap enough
Save as few $ and be out $1000 should the battery fail? Not worth it, and considering the 3rd party battery would not be covered under any warranty. At least they are not as expensive as the Phantom batteries.