Battery replacement

Apple does not sell iPhone parts. If the $49 charge for a new battery is too rich for your blood, maybe you should re-think whether you really need a smart phone at all.

To expand on Tim's excellent advice, any cheap battery will be either used, or non-Apple. Lithium ion batteries are not something on which you should scrimp. If they go bad, they explode. Also, unless you have experience working with small electronics, it's quite possible that, if you try to change the battery yourself, you'll damage the phone.