At the time of this writing there are no shutdown tile apps in the Windows 8 Store. Window's shutdown.exe can be found in %windir%\System32\ (typically, C:\Windows\System32\). Double-clicking on that will start the shutdown process.
Or one workaround involves:
- Create a new shortcut on the desktop. Right Click -> New -> Shortcut
- Point it to "shutdown /a /t 00"
- Pin to the taskbar and/or the start menu for one-click access
~ Simply Advanced ~