BattDepot, iFixit and eBay sell replacement batteries for cell phones on their websites. BattDepot offers a range of replacement batteries for Blackberry, Nokia, HTC, LG and Motorola models. iFixit offers replacement batteries for the Apple iPhone.
BattDepot also offers replacement batteries for Samsung, Sony and Sony Ericsson phones. iFixit not only sells individual batteries, but it offers kits that come with all the tools required to replace the battery. Various replacement parts for a wide range of Apple products are also offered on iFixit, as well as step-by-step guides to make your repairs as easy as possible. eBay hosts a large number of replacement batteries from a variety of sellers.