how to find Contactus page url(hyperlink) on any website ...

Hi there

You might need to use the sitemap if there is one available (Google is using the site maps to index pages too).
You can find information about sitemaps and how they work under http://www.sitemaps.org/[^].
You might find some additional information in the Google web master tools under http://www.google.com/webmaster.

cheers,
Marco Alessandro Bertschi