Low fat cheeses are still sources of calcium, but less concentrated.
Half a cup of cottage cheese (4% or less milk fat) and 50 grams of ricotta cheese (10% or less milk fat) are sources of calcium, providing about 8% and 12% respectively of the Recommended Daily Intake (RDI) of calcium. Partially-skimmed mozzarella cheese is a good source of calcium (21% RDI per 25g). In contrast, most other semi-soft, firm and hard cheeses are an excellent source of calcium (25% or more of RDI).
To learn about other sources of calcium, check out Calcium-Rich Foods.