Felix Bruns is a simple website that list most if not all of the known Apple iOS firmware for iPod Touch, iPhone, iPad and other versions of the iPod ( ex: iPod Classic, Mini, Nano, & Shuffle). They also list the firmware for Apple TV.
After downloading the desired firmware below, you can shift-click (PC) or option-click (Mac) the Restore or Update buttons in iTunes. A file dialog will open and let you choose the downloaded ipsw-file.
Note: Your browser or download manger may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.