If you don't have any ricotta on hand, you have a few options. First, you can try your hand at making homemade ricotta cheese using cow's milk and an acid like white vinegar, lemon juice or buttermilk. Second, while I haven't tried them myself, there are some swaps that *should* work here: farmer's cheese, fromage blanc, cream cheese or mascarpone. If you have cottage cheese on hand, you can also try blending it to make smaller curds before swapping it in.