Where can I get a signed iOS 8.2 .ipsw file for iPhone 4S?
Apple stops signing the old OS within a week or so of a new one coming out, for all devices that can support that OS. 

After that, there's no way to go back, unless you saved your SHSH blob from the prior version's installation.

You should be able to get Xcode 6.3.1 from Apple Developer or update from the App Store.