According to All Recipes, 8 ounces of farmer's cheese is successfully replaced with 8 ounces of dry cottage cheese or 8 ounces of well-drained creamy cottage cheese. Fresh ricotta or havarti cheese also make excellent substitution choices.
The Wisconsin Cheese Food Service offers a useful tool to determine substitutions for any type of cheese. Although some of these exchanges may slightly change the taste of a recipe that requires farmer's cheese, the texture of the alternate cheese is compatible. Cheese experts agree that harvarti, a Danish semi-soft cheese, is an unusual substitute for farmer's cheese, but its texture is smooth and its taste is buttery with a slightly sharp tang.