Take that battery out of the MacBook immediately to prevent damage to your computer. Do not use it again.
Apple sells replacement batteries for the MacBook for about $129 (in the USA).
Other users have reported being able to purchase 3rd party batteries off of eBay for $20-30.
Best of luck.
If you have a MacBook with the removable battery you can get a new third party battery fairly cheap on eBay. Every MacBook from 2006 through Mid 2009 except the Late 2008 Aluminum Unibody and the Late 2009 and 2010 White Unibodies use the same A1185 battery.
Or for $129 from The Apple Store http://store.apple.com/us/product/MA566LL/A?mco=MTY3ODQ5OTY
There’s one problem with running your MacBook with the battery removed. MacBooks and MacBook Pros automatically reduce the processor speed by about half when the battery is removed.