How do I acquire SDKs to build target pre-8.0 iOS?
You don't need previous SDK to build for iOS7. Change Deployment Target in Xcode. 

Here are differences between Deployment Target and Base SDK: http://www.raywenderlich.com/42591/supporting-multiple-ios-versions-and-devices


  Base SDK refers to the newest version of iOS that is capable of running your app. To check which base SDK you are building your app against, simply open your project file in Xcode and check the setting under Build Settings -> Architecture(...)
  
  Deployment Target refers to the oldest version of iOS that is capable of running your project. To change your deployment target, open up your project file in Xcode and check the setting under Build Settings -> Deployment(...)


Base SDK can be changed here:


Deployment target can be changed here:


Check this answer to add earlier devices support.