[bblough]

I didn't replace the battery, but back when I used mine regularly I bought an external battery specifically made for it.

The company that made it still has its website up, but there doesn't appear to be a way to order. So they may be out of stock. But you could always try to contact them, I suppose.

In a similar vein, I guess you could always get an USB battery pack and a thin USB cable that's long enough to reach to your pocket. I don't think it's ideal, but it might be better than nothing.