Nope you won't find it there

....Posting in only one locale and that being in line with the device....the platform ...or the relevant issue is suggested to help you better.
It is easier to help and it is in line with protocol ...
I suggest posting in the n800 device subsection of the forums.
People with the same devices who use them ...are able to help you better there. Don't expect to find answers about bananas in the rutabaga aisle

Not to crowd the general section any more than necessary this link does work

http://skeiron.org/tablets-dev/nokia_N800/

...the last Diablo download on the bottom is what you want....click download ...save to your computer...if you use linux there is the flasher...if you use windows...nokia software updater program is what you want...there is always the comprehensive

http://wiki.maemo.org/Updating_the_firmware#N800.2FN810.

... Nokia itself doesn't support our devices anymore...we the community do...
You may have to dig online for a copy of nokia software updater...if you can't find a copy on the web...give a shout....someone will.
instructions for flashing may initially seem like martian if you are new to the devices...but given a couple times doing it...it becomes old hat ...and you can do it in a few minutes.