A good year ago, I picked up a 10k one from Walmart--off the shelf, and impulse purchase. I didn't pay attention to the brand--it was on sale cheap, maybe $10? It has been convenient in that, when the phone has been drained, I can plug it in, drop the phone and battery in a cargo pocket, and find it charged rather shortly via the 2.1 amp socket. Seems like I'm using it twice a week, for the last three months, now.

No affiliation, just a satisfied customer...