The fastest way I know to find the icon is to simply copy and paste the following bold text into Terminal.app, an application you can find in Applications -> Utilities on your Mac:
$ open /System/Library/CoreServices/Dock.app/Contents/Resources/url.png
That's the icon you seek. Now, to use it...
In your browser, go to a page you want to bookmark - and drag the little icon out of the Address Bar (or the tab for the page if you're in multi-tab mode) onto the desktop.
and when you drop it, it'll convert into a little page with an @ on it.
Now just change the icon, which can be done by going back into Preview and choosing Edit -> Copy, then going to the desktop shortcut, clicking it once, then choosing File -> Get Info.
Now click on the icon in the top left of the Get Info window and, oh so subtly, you'll see it get a small highlight. That means it's selected. Choose Edit -> Paste and it'll change to the spring icon, both in the Get Info box and on the desktop.
Now just drag it onto the Dock itself and drop it where you'd prefer.....