You can buy it from the apple online store. All you need to do is flip the latch on the back of the computer, take off the back plate, pull the tab on the battery and replace it with the new one. (while the computer is shut down of course).
It's $129, which is steep, but considering that 3rd party batteries are generally junk (bad battery life, poor longevity, swelling and heat issues, etc.), it's worth it to just buy the same one from apple.
yeah you're best bet is through your local classifieds or ebay but you are taking a chance.... you don't know how many cycles... or if it is a bad battery or when you get it that it only holds a charge for 20 mins....but sometimes you'll find a good deal