Try Samspade, a nifty little proggy.... or here: http://www.whois.com/

Just out of curiosity, why do you need to contanct an ISP? The only reason I ask is because sometimes when you think you are getting Emails or attacks from a certain IP, they are spoofed, or proxy servers, ar any other number of circumstances, and you are not really getting the ip you want...

lol..I agree with the posts above....I wasn't quick enough.