You can copy favicon of any website by using this URL:
http://www.google.com/s2/favicons?domain=www.yourdomain.com
Just replace the bold letters with the domain name.
For Example:
If you want to copy Favicon of www.sangkrit.net then write it like this in address bar and press enter:
You will be directed to a page showing favicon of the above given website.
Save it by Ctrl+S on your keyboard.
Use it wherever you want.