Allbuy offers the XIAOMI 5000mAh Lithium-Polymer Power Bank for $9.98 USD delivered (Silver or Pink), down from the normal price of $19.18. Alternatively, you can buy this same item from Tmart which offers the product in a number of other colours (Blue, Rose, Gold, Silver, and Black).
This model Xiaomi is super thin, and about the size of an iPhone. I have one already, and it lasts/charges better than the similar size JUSTONE 6000mAh priced at US$15+.
It should be noted that it may take a month or so to receive item so probably wouldn't order if you are in too much of a rush. Allbuy claims that they changed shipping agents and it will now take 15 to 21 days. We shall see.