The official currency in Croatia is Croatian kuna (HRK). If you need to change money in Split, the recommended places are the exchange offices in the city center as they offer the best exchange ratio for any of International currencies. Among these, I usually recommend the Vukorepa exchange office, which is the most convenient and easily accesible, being in the heart of Split old town (city center): address: Mihovilova širina 9 (at the Fruit square in the old town; in Croatian "Voćni trg").
Exchange office Vukorepa, Mihovilova širina 9, Fruit square (Voćni trg), Split city center.
Where NOT to change your money?
Recommended: DO NOT change money at the airport, at the bus or train terminal or at the harbour's exchange offices; the exchange ratio at these places are far more disadvantageous comparing to other exchange offices in the city center. If you haven't already done it, also, do not change your money in your country, neither in the banks and/or at the exchange offices.
To check the approximate exchange rate you can get in Split for your currency, consult first the official currency exchange ratio of the Croatian National bank (Hrvatska Narodna Banka): Exchange rate official list. It will give you the closest evaluation of the exchange ratio you can get in one of the exchange offices in Split.