"Do you know where to buy a good dictionary?"

There is a problem with this exercise. "Добрий" should be accepted. The word "гарний" actually means good-looking/beautiful.

As a native speaker I'd rather use "гарний" or "хороший" instead of "добрий" in this situation, since "добрий" would mean "kind" almost in all instances. One exception that I can recall off top of my head, is when you refer to a unit of measurement and/or food, for instance "добра тарілка борщу" (meaning that the portion is quite large) or "добрий часник" (meaning that it has good taste or size).