I personally would use shoulder, cut it up into cubes, and let it cook down for a few hours, until it’s soft. Any kind of stew, I look for shoulder. It’s usually one of the most flavorful cuts of any animal, but it does require longer, slow cooking. Curry is perfect for this.
I should add, leg will work well, too. I prefer shoulder, but leg is often easier to find (though usually a good bit more expensive.) If you live anywhere with a Middle Eastern or South Asian population, check out their meat markets and you might find all the cuts you want at amazing prices. The Halal meat market in my neighborhood has lamb carcasses hanging in the refrigerated area, and shoulder is something like $2-$3/lb.
The easiest thing to get is a boneless leg. Lamb stew meat usually comes from the shoulder. I’ve found cross cut sections of the whole leg work well in dishes cooked in liquid, though I don’t know if that’s an East Asian style.
Update: Couldn’t make the recipe tonight, too busy chauffeuring kids. I see one store has leg of lamb on at $7 a pound. Sounds expensive, but I will only need a pound for the amount I’m making. If I cube this up it should be good, no?