Welcome 🖐
Lucky indeed.
As Dan pointed out, any 7.4v LiPo is good. Look for the highest mAh that will fit in the battery space. (Usually around 1200mAh)
Check here: LiPo Battery for RC Cars, Planes & Boats | Quality LiPo, Low Price | HobbyKing
and here: 7.4v Airsoft Battery Packs (vapextech.co.uk)
I think this is a good one: Airsoft 7.4V 2600mAh 25C LiPo Cranstock battery with case LP305 Vapextech
The connector may need to be changed depending what the gun has. (Probably a clear one called mini-Tamiya)
You'll also need a LiPo charger.
Look in those shops and also try a search for 'Li-Po charger'
Good luck