More or less random pick from Amazon, $18.99 before coupon. It can charge from USB or solar (slow) and output up to 3A at 5V. It’s rated at 38,800 mAh.
FRAUD ALERT
This isn’t what it claims to be. I should have done the math before buying.
According to the product description, this Nuynix PS21K battery pack, including case and solar panel, weight 8.3 ounces, or 235.3 grams. According to multiple sources1 the maximum energy density in commercial lithium polymer batteries is 200 Wh/kg (some as low as 140). For this battery, the theoretical maximum energy capacity would be 0.2353 kg × 200 Wh/kg = 47.06 Wh. At 3.7V (which is the specification on the product label), this comes to 12,732 mAh, three times the advertised capacity.
Over the years, Amazon has been plagued by fake batteries and fake memory devices. Don’t get ripped off.