It comes on your iOS device, the upgrades are free -- after all,it will only run on Apple's devices and Apple seems to feel under some obligation to keep their devices updated through several generations of firmware. Contrast this to the Android users for whom "what it shipped with is good enough for those peons" seems to be the common attitude to updates.
At one time, you needed to use iTunes to update your iOS device -- the only time I ran iTunes -- but iOS now has the ability to update itself over a WiFi connection. For obvious reasons, Apple does not want us downloading a 500MB or larger installer over the cellular networks.
There are also multiple sites that list the links to the appldnld.apple.com download links for various flavours of iOSx. I grabbed the 5.1.1 version of the firmware for my iPad before applying the iOS6 update just to give me an easier backout if I was unhappy with iOS6.
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