Get one off greedbay (opps I mean ebay) used for around $75. Ask how many charge cycles the battery has, this will tell how many times the battery has been charged.
IE.. if the battery has 50 charge cycles, then the battery has been charged 50 times. The less charge cycles, generally the new the battery (unless the macbook is always plugged in).
Is your macbook still under Apple's limited warranty, if so call Apple for a replacement.