The key factor for these is the mAh rating, that is a measure of the capacity of the powerbank's internal battery. The iphone 5s has a 1560mAh battery, so a powerbank with a little over this capacity should recharge it fully (there will be losses and battery capacities fade over time)
You can also buy these directly from the Far East via AliExpress or DX.com, most of the EBay sellers are buying from them and adding a significant margin. Like a lot of this stuff, quality is very variable, so check feedback carefully before parting with your money.