EC3 is the correct connector for the drone side.
Also XT60 would be the most common for the other end.
This battery fits inside nicely.
ZEEE 3S 5200mAh-XT60 Plug Lipo Battery
Search above title on eBay. I do not sell this battery on eBay only trying to be helpful with example of a confirmed battery type that works well with our Q500 drone.
In photo below is a great holder for the EC3.
When you close the door it prevents the EC3 connector from accidentally coming unplugged during flights. It's also small enough to still be able to close the battery door with the battery, cord and adapter all fitting inside the battery bay. Battery in this photo is similar to the one I mentioned above.