Apple has put the download of iOS 5.1.1 Final in iTunes for iPhone 4s, 4, 3GS, iPad 1, 2 and then iPod touch 3g and 4g. But you can download this iOS 5.1.1 firmware through direct links that I provided in the end of this post.
A friendly advice: for those who wish to jailbreak or unlock their iOS 5.1 device, it's recommend that you don't upgrade your existing iOS firmware version to this version, as your iOS 5.1 will be patched with it. And for A5 based device users, it's double recommended not to put their hands to this iOS 5.1.1.
Anyway here is a list of iOS 5.1.1 firmware that you can grab according your device model: