Try checking the service manual at
http://h10032.www1.hp.com/ctg/Manual/c00246219.pdf
I couldn't see much about the battery but it gives detailed disassembly instructions. It will either be a standard lithium button cell that most electronics retailers can supply off the shelf, or it will be a proprietry shrink-wrapped one with a fly lead.
If it is the latter you are probably best to order a replacement from HP quoting the part number on it as lithium batteries can be tempremental at best and you don't want to end up with it starting a fire.