Muffaletta Bread in Manhattan
focaccia works just fine - i use trader joe's rosemary focaccia and it blends quite well with the meat and olive salad.
i ordered my central grocery olive salad from this site, and they also have muffuletta bread if you're looking for something more authentic!
New Orleanian living in NYC here.
I've never seen a muffuletta loaf in a NYC bakery, and am guessing that local NOLA-style restaurants either substitute other breads for their muff loaves, or bake their own. A New Orleans muffuletta is a BIG sandwich -- 9 or 10 inches in diameter.
I think a fluffy focaccia (around 2 inches high) split horizontally is one substitute that might work nicely. But I wouldn't sweat the bread too much. As long as your filling ingredients are solid, I doubt you'll hear any complaints!
If you're feeling ambitious, you could try baking your own bread. Here's one recipe:
And Chuck Taggert has a good recipe for the all-important olive salad: