OP if you are willing to come down to Bucks County there are several places that have fresh lamb that would include lamb ribs. You might also try one of several of the butchers at the Stockton Farmers Market.
aklein - The best lamb ribs I ever had were at Drew's Bayshore Bistro. They had them a month or so back as a special. I would give the restaurant a call to see if and when they will be available. Good luck.
This Sam Sifton article suggests any butcher should be able to get you lamb ribs. Of course, what's easy for Sam Sifton (at the the time he was the food critic for the NY Times) may not be so for the rest of us.
I suggest you try Wegmans in Manalapan. I've not seen lamb ribs in the display case. However, in the past, when I've asked for something they don't normally stock, they've told me they'd special order it for me.