Jump over to this website http://www.felixbruns.de/iPod/firmware/ which has every iOS for every model of phone. Simply find the correct model for your phone and click on the iOS version you want and download.

Once downloaded, follow the instructions on the top of the site (copypasta below)

>How to restore or update your iPod/iPhone/iPad:

>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 may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.*