possibly this ...never tried it soooo....caveat emptor...
http://www.ebay.ca/itm/NOKIA-Battery...item5663732720
but I would trust this more...
esp. for the price...
http://www.ebay.ca/itm/OEM-GENUINE-N...item43ca9636de

currently there are not a ton of n800 batts available on ebay...but that changes all the time...

up to you..
there is also the possibility of taking a beaten battery and turning it into a dummy batt and using say this...http://www.ebay.ca/itm/141082066753?...84.m1436.l2649
and then you would be free of batteries ...

As far as "how" to do wizardry hocus-pocus al-a-ma-focus stuff...
[or as we here call it..."the norm"] if you are not used to linux or maemo for that matter...
there are many "how to's" ...
hell I did a pretty comprehensive how to...
from flashing ...step by step...for the compleat "newmo" [tm, patent pending] like you... to go from absolute ignorance and yet still end up with the zippiest most tweaked out device that can possibly be done...
in fact to make it even easier...
instead of reading..
I included numbered video tutorials..
there are plenty of choices for you in our forum and wiki here...