Installing a specific iOS on a new iPad
There is a lot of information covering this exact topic in a question that I asked a while back:  https://apple.stackexchange.com/questions/65952/how-to-upgrade-iphone-3gs-to-ios-5-now-that-ios-6-is-out

The accepted answer provided me with this link: http://www.felixbruns.de/iPod/firmware/

Hope this helps!