should help you out I am facing the same issue on Xcode 7.3 or Older version of your Xcode and my device version is iOS 10 or newer version of your OS. This error is shown when your Xcode is old and the related device you are using is updated to latest version.
Xcode 7 with iOS 9 device Error: device unavailable (Could not find a developer disk image)
hope this fix your issue If you are using the Xcode 7 GM build version 7A218, the chances are that you accidentally downloaded iOS 9.1 (13B5110e) as opposed to the iOS 9 GM build (13A340). I did the same thing don't worry. They are too close to each other on the downloads page :0 You can check which version you have on your device by simply going to your device Settings > General > About > Version
Could not find Developer Disk Image - Xcode 7.2 & iOS 9.2
This might help you When I installed El Capitan (got rid of the "This Core Storage operation is not allowed on a sparse logical volume group" error by activating FileVault on my Mac), I saw an Xcode dialog box saying it needed to install additional components. I am unsure whether that dialog box was there before I upgraded OS X or not, as it might have got lost amidst other application windows. Either way, that seems to have installed the disk image needed to run apps on the iPhone.
How to Fix Xcode Error “Could not find Developer Disk Image” on iOS9.2
I hope this helps you . I don't think Apple has ever released the developer disk images separately so I can't imagine they would in the future. Solving the download problem is probably easier, don't use App Store because it's fairly miserable about downloading on a bandwidth limited connection but instead visit https://developer.apple.com/download/more/. They have the actual files hosted there so you can get them with a more appropriate tool, you'll know they're definitely from Apple as well.