Anker is a good name and you'll hear it thrown around a lot. I have a 6000mah and 10,000 mah, and just gave away the 10,000 to my daughter in law that didn't have one. I'm currently on the lookout for a god deal on something in the 20,000 range. I'll probably never need that much, but I'll probably never need more either. I have found several for $35-$40, but I'm holding out for a sale.
If you're not in a hurry, go to slickdeals and set up an alert for a 10,000 mah battery (that's what I'd consider as a minimum). Within 2-3 weeks you'll find a bargain almost guaranteed. If you are in a hurry, here's a real nice slim Anker for $30. It's one of their newer ones with the PowerIQ technology, and $30, sold by Anker through Amazon. You can definitely find cheaper, but that's also a fair price for a good battery, and I didn't have time for an exhaustive search this morning. Best of luck to you.
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