are you talking about the internal, CMOS battery on the motherboard
or the external, removable "workin" battery??
if it's the former, then i would think on laptop not from the stone age, that it would be some sort of pill battery that is replaceable, but Apple might have kept that part backwards since they generally don't like people doing DIY on their stuff
and if it's the latter, i would think there are plenty of sources for batteries, like this one
http://www.bizrate.com/batteries/oid535920599.html
and even if it is the CMOS battery, i think it would work by simply having a working main battery, that should keep the cmos running too
the CMOS battery is only there, to remember it's own settings when all other power fails... you can even do without, but it would require entering the settings , each time you boot after a main power failure(AC or battery)