https://www.amazon.com/dp/B099N3S2FR?psc=1&ref=ppx_yo2_dt_b_product_details
These are the ones I got. There are other options available, but I recommend whatever has the highest mAh rating without having unnecessary features, like bluetooth phone capabilities. These are 3500mAh and reasonably priced with 3 heat options (low - green, medium - yellow, high - red). They have a simple one-button interface and no weird gimmicks.