From experience, I would buy an original Sony battery - through ebay or amazon. By time, I'm learning that in photography (as in other things) the cheaper ends up to be more expensive; as it would need to be replaced sooner.
Lately, I bought 2 generic batteries for my sony a580. I admit that they were cheaper, but one of them seems to loose the charge faster and does not display the remaining percent. The other works all right.
Sony α580 dSLR
Tamron 17-50 2.8, Sony 18-55 kit lens, Minolta AF 50mm 1.7, Minolta 28-105mm xi zoom, Minolta 35-70mm 3.5-4.5, Minolta 75-300mm 4.5-5.6
My short photostream: ivandegi