Try www.dealextreme.com

I brought a Japod BL-5J Replacement 3.7V 1350mAh Li-Ion Battery for Nokia 5800, cost me around $9 or 6 english pounds.

Delievery takes a while though i was waiting for about 2weeks but you can track it online.

Performance wise, i think its slightly better than the original nokia. I read a few battery test forums on here, before purchasing it, and tbh it was worth! The battery ain't a fake despite its appearance lol.

Nath