HRK-KUNA is Croatian currency. 1€=aprox 7,5 HRK
In all places such as airport, harbour, marinas, shopping centar, or in the centar of city there is ATM where you can get cash in Croatian currency KUNA (HRK).
All shops accept paying by any credit cards or in cash but in HRK. Paying in Eur is not possible.
The best way to change your money is in banks that you can find in any starting base or in a bigger places on islands:
Island Brač: Milna, Supetar, Bol /Island Hvar:Jelsa,Vrbovska,Starigrad, Hvar/Island Vis: Vis,Komiža/Island Šolta: Rogač/Island Korčula: Vela Luka, Korčula/Island Mljet: Sobra.
Mind: smaller islands as Šćedro, Biševo.. have no shops, banks etc but maybie you will be able to buy fresh fish from local fishermen, or domestic honey so cash will be always still needed.