Portable batteries from Choetech, Krome, Endefo and Hoco.
Capacity in mAh translates roughly to phone charges: 10,000mAh is about two full charges, 20,000mAh about four. Higher capacity means more weight, and above 20,000mAh some airlines start asking questions.
Output wattage decides how fast it charges, and it is the number most people ignore. An 18W or 20W Power Delivery output charges a modern phone several times faster than a 10W bank of the same capacity.
If you also carry a laptop, look for 45W or higher with USB-C PD — a bank that only does 20W will keep a laptop alive slowly rather than charging it.