Where can I find original MacBook Pro batteries?
Apple don't sell batteries as spare parts to customers, and the Unibody MBP's battery is not 'user-serviceable'. Hence the funny tri-cornered screw heads. Anything you find on a third-party retailer's site is likely to be 'grey', even if claimed to be Apple-branded.

(Having said that, I have bought several of such batteries for Unibody MBPs and they have worked well with no problems.)

If you take your MBP to an Apple Store, you should be able to get it replaced, but obviously Apple will charge you 'some money' for this.