I think the N800 uses the BP-4L battery. Dealextreme has one of those made by Pisen for around $7. I'm using one in my E63 and it works pretty well, comparable to the original when I bought it about a year ago, maybe somewhat decreased in capacity now after the past year of use. I just bought another one but am not using it yet. Japod batteries from DX are even better than Pisen but they don't have a Japod in that model. I used a Japod BL-5C in another phone for a while and it worked great.

Edit: Looks like the N800 take the BP-5L rather than the BP-4L. DX has one for $9 (ouch) that I guess I'd give a try: http://www.dealextreme.com/p/45353