Apple currently only ship the latest SDK's with Xcode, including updates to Xcode.
This can make updating legacy apps difficult.
So here's a collection of the SDK's obtained from the Xcode installation DMG's.
Currently available:
- iOS 4.3 (Not recommended as it's very old)
- iOS 5.1
- iOS 6.1
To use these SDKs, drop each of the .sdk folders you need into: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/
Restart XCode and you should be good to go.
Disclaimer: No warranty or support given.