Here's my experience. Buy something from eBay or Amazon.
Well, we've bought a lot of batteries from various people I've been harvesting laptop batteries for the 18650 cells to put into phone recharging backs so we can play Ingress for effectively limitless hours, and for eCigarettes. That's given me a look inside them, and what condition the actual cells are. Leftovers, I sell to friends and friends-of-friends at cost.
The recharge packs I have take 4 18650's, so if I get 2500mAhcells, I have a 10000mAh pack. I went with carriers that have a physical on/off switch, rather than the soft switch like the Anker has, so they can sit a long time without discharging. I haven't needed to change batteries on them yet.
Generally, I buy from eBay. I'm looking for the higher cell counts, and aiming for about $1 to $1.50 per cell. So a 8 cell pack I want to spend $8 to $12 on.
When I crack them open (always more work than it sounds) they all have the standard overheat sensors, which was the concern before about exploding batteries. They have all been wired well. Out of say a couple dozen packs, I received one that had a dented cell in it. It didn't hurt the performance of the cell, but since it was dented, I refused to use it or give it to anyone. Some of them, I've damaged the wrapper, so I re-shrink wrap if I'm in urgent need of them, or I dispose of them.
Regardless if it says on the listing that it's an OEM or 3rd party pack, almost all of them have had no-name cells in them. I did get a few true Sony, Panasonic, or Sanyo cell, but they are rarer.
They've all tested out to be the listed capacity, and they all have worked at the expected life expectancy.
The only big exception was the battery for my old cell phone. It originally came with a 1400mAh battery. The only cheap seller listed 1600mAh for about $10/ea. I used them, and they were fine, but they only lasted as long as my original battery when it was new. When they finally started failing, I pealed the stickers off, and the original markings showed they were 1400mAh batteries. If I had been paying extra for the extra capacity, I may have been upset. Since I just needed batteries that worked, it didn't matter much.
I played Ingress a *lot* with my phone though that period. That draws a lot of power, so I kept a couple spare batteries in my pocket all the time so I could swap them as needed.
My new phone came with a much larger battery (part of my selection criteria), and I don't play as much. I let it charge in the car when I'm driving. If I'm walking, still carry the external pack, just in case I need it.
So.. Pick something cheap on eBay. Look for listing saying they're "new". Don't expect a higher capacity batter to be any better than the original battery. Since you're looking for cheap, you can generally afford to get a spare.