As has been mentioned, most of the lamb in the US is imported -- and mutton is imported, too
There's a giant consumer of mutton in western Kentucky around Owensboro. (it's the local standard for barbecue, and past reports put Owensboro near the top of the list for both for per capita number of and spending at restaurants)
As we now have a lot of smaller sheep farms making cheese and wool, especially in mountainous regions, I would suspect that there are people eating mutton, but it may be that they're keeping it for themselves. There's also a possibility of selling it to ethnic butchers, as sheep and goat are popular in some european, asian and middle eastern cuisines. (I know I can get rabbit at one of the local Italian markets, and goat at a local asian market)
Although I suspect that there may be some labeling issues in selling mutton as 'lamb' (much like people would be upset if you sold beef as veal), I've heard that some butchers who make Merguez sausage will use a blend of mutton and lamb -- the spices mask the possible gaminess of the mutton, but there's still lamb in it so they can label it as such.
I'd suggest looking in your area to see if there are large farmer's markets -- there are often farmers selling meat, cheese, and yarn there. If you find someone selling sheep products, you can ask them about obtaining mutton or hogget (1 to 2 year old sheep).