I bought a BL-5J clone for US$3.72 each delivered.
http://www.dealextreme.com/details.d...691~r.60561251

Capacity is limited, but for the price it makes a good emergency backup.
I got less than 12 hours, including overnight, before it died with little warning. Maybe an hour of music, a little web. Its such a power-hungry phone, that you need as much battery capacity as you can get.
The cell is protected, and it even has a real hologram, but not as nice as the true nokia one.

(disclosure: referall code gives me 1c/battery:-)