Lardo in NYC
If you are just looking for fatback, there are prepackaged ones in Citarella, Whole Foods, and most grocery stores.
If you are looking for "lardo" (very specifically placed in the counter and not prepackaged), I have seen them in DiPalo and Dean & Deluca. I am sure Buon Italia has that as steeltowngrl mentioned.
Okay...lardo...if you're interested in finding a good lardo at a restaurant, the one at Del Posto is delicious. I'm generally not a big fan of it but I was seriously depressed when I realized that the last of ours was gone. Cesare Casella offers a nice one too at Maremma.