Greenworks 24V Battery 2-Pack, 4.0 Ah Batteries, K2D8NNAA2

If you want genuine GW parts (to keep your warranty valid) then this pair from Walmart is a good deal - they are compatible with your tools and charger, and double as power banks If you want to experiment with aftermarket (potentially risky) then these from Amazon seem like a deal and might let you finish mowing without having to swap batteries