Currently Being ModeratedJan 2, 2014 7:00 AM (in response to prabhakant)
There are no user serviceable parts in any iPhone and Apple does not
Any parts you find for sale are from damaged or stolen iPhone or third
party knockoffs. Any damage from these parts or attempts to install
such parts is your problem.
Contact Apple for availability and cost of battery replacement for your
iPhone 3GS. You probably would spend less to upgrade your iPhone to
a later model.