The battery for my Macbook Pro 17 died, Serial No. W02271DLZ9DA, Model No. A1189 10.8V 68Wh. I was told at our local Apple retailer that that battery cannot be bought from them. I have found several comparable aftermarket batteries online from $20.00 on up, but have been told these could harm my computer. Can you tell me if that is correct or not and if so, what are my options? I have been using my computer with electric, but my cord has been unplugged while working and the risk of losing my work is great.