It used to be hard to find a reasonably priced USB Type-C battery pack, but it's getting downright cheap these days. The Aukey PB-XN10 power bank is usually a good value at $23, but it's even cheaper today with a coupon code. It can be yours for a mere $17.01 if you act fast.

This Aukey battery has USB-A, microUSB, and USB Type-C ports. The micro plug is only for charging, but you can charge via Type-C as well. The USB-A operates at 5V 2.4A, and the Type-C can do Power Delivery at 5V 3A (15W). The maximum output is 15W, so you can charge two devices if you don't mind slower speeds. The 10,000mAh capacity is enough to charge most phones three or four times.

At checkout, use the promo code AUKEYR45 to apply a $5.98 discount. That brings the price down to $17.01. I don't think you'll find a reputable Type-C battery of this size any cheaper. This one also has Prime shipping. It's hard to go wrong.