How Can I get an offcial OEM iPhone 4S Battery?

Hi, guys, my iPhone 4S is out of warranty and the battery drain too fast, I plan to replace a new battery for my 4S, but I don't know how I can buy an real offcial OEM iPhone 5 Battery(I don't want to get a fake battery (my friend told me the fake battery may explode),
Any recommends? :confused: