If you're referring to the TBP12 handheld, the screw-on battery cover bears the Line 6 Product Code 30-27-0434 and will be available from the Line 6 agent or Service Center nearest you.

I doubt that they would keep such things in stock, so you'll need to allow some time for shipping.

You may also find the following discussion thread (and the even earlier thread referred to in it) of interest:

http://line6.com/support/message/357087#357087