I googled the fullriver battery company in China and found and found this page where they picture the original Pebble. http://www.fentbattery.com/en/Cylindrical.asp?id=454&pid=108
After a little poking around their site I got to this chart:
http://www.fentbattery.com/Upload/2016050710440332332.png
I haven't cracked open my Pebble steel to measure the battery yet, but I did google a couple of the model numbers like this : "043030C battery". This resulted in a string of China import sites (aliexpress, dhgate, etc) that offered to post me one of these.
I guess if you are willing to measure your battery dimensions, and order the corresponding battery from china you could finish up with a cheap replacement battery. I haven't tried it yet, buy have a depleted battery too by the looks, so interested in how you get on. Who knows you might even manage to squeeze a 300mAh battery in and get 14days out of your pebble?