The EforTek NP-FW50 Replacement Battery (2-Pack) and Charger Kit for Sony NP-FW50 is a great way to get replacement batteries for the Sony A7II mirrorless camera and other Sony cameras that use the NP-FW50 battery. The original battery that comes with the camera has 1020 milliamphours of capacity. The Sony store offered to sell me a replacement battery for $80. On Amazon.com I found the EforTek bundle that includes two batteries of 1500 milliamphours each, a wall charger, an adaptor for European style plugs, and a car "cigarette lighter style" plug in for the charger. The cost was under $25 for the whole thing.
I'm currently running the camera on one of the replacement batteries, and it works fine.
It is said that the A7II is a battery hog, and so far this is indeed the case. So I highly recommend getting two or more spare batteries with charger if you own this camera. The batteries charge up pretty fast, too.