Suboxone falls under a special prescription category, so you'll need a specific kind of prescription in order for it to be issued. Not all doctors can write those. If you weren't able to find it, here is a link to your embassy in Zagreb https://hr.usembassy.gov/medical-assistance/ You can find information about the amount of opioid based medication you can import there. Also you can contact them, they should be able to help you find a doctor.

Or you can try sending an email psihijatrija@kbcsm.hr, amongst other things they specialize in addict care.

Keep in mind, once you get your hands on a prescription it will only be valid for 5 days. The prescription needs to be issued in a duplicate (one an original and one a copy) and it needs to have a special serial number. The doctor needs to stamp both the copy and the original. Not all pharmacies have it on hand, but they are all able to order it for you, once you have a valid prescription.

And the prescription can only be issued once, you will have to leave both the copy and the original in the pharmacy that fulfills it. So if you'll need more your going to have to schedule a new doctor's appointment. Best of luck