I bought the Anker 3000 mAh battery and cover for around $12.50 from Ebay. The battery works great and adds a ton of life to the unit. Couple of things to consider:
1. Do you want NFC? Many of the batteries don't come with NFC. Some do though.... If it's important to you, just make sure the one you buy has it.
2. The larger the battery, the larger the back cover needs to be. For example, the Samsung/Verizon extended battery and door (which I think is 2100 mAh) is barely noticable size difference and my phone case still fits. However, my 3000 mAh is huge and there is no way my case would fit. But, I just use my 3000 mAh on occasions where I know I'll be away from a charger for the entire day.
As far as the brand goes... unless you see bad reviews I would say they're all probably about the same. Check ebay and amazon for the best deals from what I've seen.