Which 15" model? There have been several, some with removable batteries, some with built-in. The Apple Store carries what you need, but you need to know which model you have, first.
Whatever you do, don't buy a cheap aftermarket one. They are junk, sold under different names by various marketing companies. Apple's is overly expensive, but it is the one that works.
There's only two of them. Go to the Apple Store, type '15" battery' (minus the ticks at the ends) into the search box, and they'll be the first two items in the result set.
If I did this correctly, the model number I found was MC 026LL/A for the MBP.
Went back to the Apple site and I pretty sure this is the correct battery I want.