http://www.techstuffs.net/p/all-apple-firmwares-ipod-iphone-and.html

Apple Firmwares:
Note 1: When you download the required firmware below, you can shift-click (on PC) or option-click (on Mac) the Restore or Update Buttons in iTunes.
Note 2: Your browser may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.