would like to find out which was the ISP of a certain web site, in a
certain date at the year 2000.
 
I found a web site which seems to be what I need:
http://uptime.netcraft.com/up/graph .
Unfortunately, the information about the site that I search was
updated only from the year 2002.
 
Is there any other site, or other method, that I can use in order to
find the information?

You can try to find the information from a possible previous version
of their site from: http://www.archive.org/web/web.php

Or you can try a generic WHOIS lookup on the site: http://www.whois.net/

Thanks; I have already used web archive, and was able to find a stored
copy of the site from the date I need, but I don't know how to find
out the site's IP at that time.

As Andrew said you can do a whois on the websites current ip address.
I suggest that you use ARIN whois located at
http://www.arin.net/whois/ This will give you the hosts current
information including the ISP of the site. It may give you the date
that the ip range was give to the isp. However, it could be the case
that the site hasn't always been with their current ISP in which case
it would be very hard to find an IP from 2000. You may be better off
just calling them and asking for the information, it would be outdated
information so it wouldn't cause a security threat...they may give it
to you.