commercial ricotta cheese.
Fresh ricotta should not be hard to find. In Manhattan, both DiPalo's and Alleva are good places to go. Eataly should also have it.
If you are in Brooklyn or the Bronx, check the Outer Boroughs board for places, since it is very easy to find in Italian neighborhoods.
As far as the commercial stuff, Polly-O is available in most supermarkets, and it's not bad.