This is the one I have and it works great. I get about 5-6 hrs of solid play time from it:

https://www.amazon.ca/gp/product/B07YPY31FL/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

I also bought this armband so I could strap the battery to my arm with a short USB cable:

https://www.amazon.ca/gp/product/B07JL74R2J/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

The battery doesn't have any clip or holder on it so the armband was the next logical option although I've seen quite a few 3D prinatable clips you can possibly use to mount it to the HMD directly.