I doubt you need one yet, but they are available on the Internet.
http://www.newupower.com/products/Battery-for-TG-UV2-Walkie-Talkie-WP0902_1177.html $14.99 "free shipping" (?)
Note, I'm not recommending these vendor because I have no experience with them.
Personally, I'm going to wait a while as I expect that the included battery will probably last for two or three years. And perhaps DealExtreme will have it available before I need one anyway.