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Apple considers Unibody MacBook Pro battery replacement a repair that users are not supposed to do themselves, so they don't show the batteries for purchase in the Apple online store, nor does Apple sell the batteries separate from installation services at an Apple Store or other Apple authorized repair facility. My son recently had the battery replaced in his mid-2009 unibody MacBook at the Apple Store for US$129, including installation, but not local sales tax.
There are other sources for a replacement battery. iFixit.com lists the battery for $119.95
http://www.ifixit.com/Store/Mac/MacBook-Pro-13-Inch-Unibody-Mid-2009-Mid-2010-Ea rly-2011-Late-2011-Mid-2012-Battery/IF163-019
A video showing the replacement process is included on the page linked above.
I've not replaced a unibody MacBook battery, nor have I purchased anything from ifixit.com, so this is not a personal endorsement, nor a recommendation that you should try to replace it yourself. You asked for information.
UPDATE: If you do not already own the #00 Phillips screwdriver, the specialized tri-wing screwdriver needed to remove the battery screws inside, or the 'spudger' or similar tool to remove the battery power cable, you might consider paying the additional $9.05 for installation by Apple (119.95 ifixit price+9.05=129 Apple price) a worthwhile price, especially if you live near an Apple Store or other Apple authorized repair facility. Of course, if you invest in the needed tools now, you'll have them for future repairs.
Message was edited by: kostby