You can usually find a cashpoint. Lots of restaurants/supermarkets take cards - including debit cards. Check with your bank if they charge you for overseas use of card. Nationwide didn't used to (have vague feeling that DH may have mentioned recently that they now do ). We usually withdraw some money from the ATM at the airport (the destination airport, not departure). IME you get a better rate than changing money at this end before departure.
Where are you going to? Might help with the cashpoint question
okay. Not a country I have any experience of. You could try Trip Advisor general travel forum for Croatia and ask a question about ATMs in the specific town you are going to. There are usually several people around on those forums who know everything
You might have to tell your bank/credit card people where you're going. My c-card won't work abroad unless I tell the provider in advance, though my cashpoint and debit cards seem to work anywhere, regardless.
And you can use this link to find the cheapest forex rate: (collection is usually cheapest if you can get to them)