I’m not sure why mutton isn’t available. Sheep are raised both for lamb, and also for wool. Once a sheep is old enough that it’s not useful for wool production, it’s usually slaughtered. But where does the mutton from that mature sheep go? I just don’t know. All I know is that it’s very difficult to find in the US.
That’s very strange to me. We had a lot of butchers in Chicago that sold goat, but none that sold mature sheep.
Have you tried checking halal butchers? I know that goat was much easier to find in the areas of Chicago that had large Pakistani and Afghani populations.