I've bought one form ebay, the seller says it's from an OEM. But Chinese sellers tend to think OEM stand for "Cheap Chinese knockoff" instead of "Original equipment manufacturer".
So I don't know, we'll see. lol
He shipped it using China post which is why I chose him instead of *******. The prices are crazy there because China post no longer ships batteries unless they are stored within a device. So that makes you wonder if it even ships, it may never arrive. Too bad there's no tracking number.
I heard about Nohon BM20 battery with real 2050mAh (some specimens even with 2100mAh if you are lucky).
Those batteries are highest quality but a little expensive, they are really good.
http://www.**********.com/af/xiaomi-mi2s-nohon-battery.html?ltype=wholesale&SearchText=xiaomi mi2s nohon battery&isOnlyAffiliate=y&d=y&origin=n&initiative_id=SB_20150705075717&isViewCP=y&catId=0
ps: there are also Nohon batteries for other phones: