Hello again!

So, I'm thinking about buying a N800 that lacks an Operating System, so I've been looking through the forums and I can't seem to find my answers.

What files or drivers or whatever they're call do I need to flash my device?

And where can I find them now? From what I've seen, there are some different links, but all are broken or go nowhere...

You need initially 3 things which I will shortly post as updates inside this answer(saves tons of post after post)

Update: This may seem like Martian to win/mac users...but ...once you do this a couple of times..it is insanely easy and quicker than explaining it
the wiki... where there is some useful reading..(be careful it is easy to read your way into what to do on the n900 instead of n810/800 ).
http://wiki.maemo.org/Updating_the_firmware#N800.2FN810
What you need:
1-A computer linux/mac/win to download and flash with ..
2- the 3.5 flasher tool or the Nokia internet tablet update wizard software tool (freakin mouthful!) ...perfect for windows users..works well on win computers up to win 7. (I suggest using admin compat. settings...ie right click on program icon ...go to properties...enable older compat.win xp for ex. and admin. ) which you can still find on the web.. I suggest making copies and zip/rar-ing them for future safekeeping back-ups.
3- the Latest Diablo O.S. bin for the n800 ..I suggest contacting some one who may know where to find said image ....
4- the usb to micro usb cable that came with the nokia. If you don't have one ...they are cheap and easy to find.
5- plug in and charge the battery up before continuing.
6-follow pertinent steps discussed to flash your n800.
7-after flashing ...your nokia is better than factory fresh .

Tweaking , adding repos and installing a variety of successive platforms...diablo...cssu...diablo turbo...etc... will provide even more options for your device. As well...cloning (copying) the entire operating system onto a sd card and expanding your storage, usability, device lifespan, etc. as well as run multiple os's (great training/ testing ground for using the same in say the n900 or upcoming neo900 or for understanding linux in general.)...as far as benefits go to updating after flashing ..the list is long...and goes on...The device is like nothing you have ever used...I've been using mine for almost a decade ...I have current toys as well. But I don't consistently use them on a daily... like the n810/n800.

Update: Instructions for flashing I posted just recently http://talk.maemo.org/showpost.php?p...32&postcount=2
you get past this...you will probably be a little anxious about possibly breaking the thing...it is pretty hard to bust this sucker...worst case...just reflash ....you will get tired of the factory install ....so once you are done farting around with it in that state...I suggest a clean reflash...then...I suggest enabling repos in Application manager (click on onscreen upper-left file manager once app manager opens....go to tools...open repos. enable unchecked extras-repo...close..let it update. immediately install root app. ...this gives immediate ability to mess with your device ) before installing all kindsa stuff...I suggest getting penguinbait's console tools ...latest.
clone your pristine n800 onto a sd card. THEN do all the updating and tweaking , ( repo list adding from gronmayer (easy quick and huge list) .....of the os on the sd card....

Thanks @endsormeans ! You definitely seem to know a lot about these devices.

Two more questions:
1- Is it possible to have, let's say, nitdroid and diablo installed at the same time, and I coul just choose one at boot?

2- Is it possible to flash it while charging? The battery on the one I'm buying is completely dead and I can't find one cheap enough...

Two more answers :
1- Yup....Now then ...some of the bootable O.S.'s have good smooth performance with few bugs or issues...others are problematic...still others can give you grief ..Easy Debian Turbo is the smoothest ...easiest ...most prob free of all the multi-boot or chrooted O.S. options and I personally see it as a descending order of ease and usability and functionality from there...Remember...that some of the os's and the methods were originally experimented with... just to prove it could be done
Trying to fit an O.S. meant for a completely diff. platform with diff hardware and making it work...even partially on a handheld is frankly a marvel IMHO. Sure NITDroid works...it has a couple of issues...Palm works...Mac works...Other Linux work...but it is about the degree of usability.

2- Flash while charging? NO. That I do not suggest doing. I suggest with flashing to do that properly....it is hard to roast your device...since you can simply flash over it...But...messing with the process itself ...mmm..- mmm ..that could give you very fascinating results that you may not care for.
Get a battery that can hold a charge....even a 1 hr charge is better than nothing and you should be able to do it...

BP-5L? You looked specifically for that in searches?
Hmm ebay nets ones starting at 7.00 but I do admit a few of them look a little sketchy...Avoid buying bp-5l from asia..Also ...avoid buying any battery alternative that is supposed to last longer...ie mugen ...In the forums here you can find all kinds of probs. ...from ok results...to dubious..to downright dangerous results with battery alternatives. frauds and fakes people have had....there are some decent ones from germany and the states and brit isles that are newer....