One good place to find repos is on the maemo wiki page for repositories. To add them, just go to the application manager, then pull down the menu, select Tools|Application Catalog, click new, and add the URL and other info as posted on the wiki page.

Now, as to kismet, if you're using the latest firmware on your N800, kismet scans for about 10 seconds and then stops. As far as I've been able to determine there is no fix for that at this point. If you're using the previous firmware, kismet works well, but video playback on the older firmware is not as smooth. See this thread, this thread, this thread, and finally, this thread for more details. I'm no guru, but it seems to be a kernel issue rather than a user-space issue, and as I said, no one has figured out a fix at this point that I've heard of.

Aircrack-ptw and tcpdump are available from various repositories. Aircrack-ng is available also, search the forums for a link to the .deb file.