where to buy a macbook pro battery?

I have an old unibody 15" MacBook Pro C2D. The track pad will not push down and from reading online it seems this could be from a swelling battery. I am fixing this laptop to sell. Now where is a good place to buy a battery for this MacBook Pro? Something directly from Apple? or are there good quality non apple batteries?

via ehMac.ca http://ift.tt/1lnDfjA