If you are looking for the latest print editions or official e-book access, these are the primary publishers:
Most "exclusive" or widely used collections are published by major educational houses. These books typically include theoretical overviews, solved examples, and graded problems. Kreativni Centar: Fizika 6 Zbirka fizika+zbirka+zadataka+za+6+razred+pdf+exclusive
It looks like you’re searching for an of the Fizika zbirka zadataka za 6. razred (Physics collection of problems for 6th grade). Fizika zbirka zadataka za 6