Olive oil has been produced in the territory that is now Croatia for millennia, but olive trees are not indigenous to the region. Olive trees were in fact introduced by the Romans.
Croatia takes pride in its olive oil production today, and you can purchase olive oil in various quantities as a souvenir of your travels. Look for olive oil at markets, supermarkets, and souvenir shops.
If you don't want to attempt to bring olive oil back home with you, do try Croatian olive oil when you're at a restaurant. Olive oil is often eaten with bread as an appetizer at meals.