I am sorry to say that I have been looking for mutton for two years, and have had zero luck. I have tried nearly every specialty butcher shop in town, as well as trying the odder , more ethnic food palaces, and everyone tells me the same thing--nobody eats it, you can't find it, they can't even order it. I am not saying it's impossible--but I have been unable to get my hands on it.
In a curry though, I'm not sure you'd taste much difference. What you want is a nice roasted leg, the kind me muvver used to make. That, is going to be a tall order unless you cook it yourself.
If you want to prepare it yourself, rumor has it that you can buy it here http://www.dairysheepfarm.com/ (they're at the Union Square Greenmarket ) although I dont see it on the website.
As for a restaurant, you'd think that there would have to be some well hidden ethnic joint serving it somewhere but I havent found it yet. Sorry. Guts may well be correct.
Union Square Greenmarket
Broadway and E 17th St, New York, NY 10003