The standard batteries were nearly worthless anyway, not much battery life, so even if you could track down new ones I'm not sure you'd really want them.
I ended up slicing one open, ripping out the internal cells, and soldering wires onto the contacts of the old battery and then onto connectors to a new power source, a big external SLA that gave me 6+ hours. Just make sure you don't get the polarity reversed - this is VERY important or you will fry the recorder IIRC.