Elsewhere in Slovenia it's hit or miss, though acceptance of credit cards is spreading. In a pinch, you can always go to a bank and get a cash advance on a credit card. Even if you plan to use credit cards for most purchases, you'll want some local currency for in-country transportation, small items and the occasional business that doesn't take plastic. Be sure to go to the bank and not to the sundries-kiosk, where rates are worse. The hotel will also change money.
Banks offer the best rates, post offices only slightly worse.
Hotels and commercial money-changers are often substantially worse but have much longer hours. If you change more money than you spend, be sure to change it back before you leave the country, since it's nearly impossible to change tolars outside of Slovenia. You'll pay double fees, of course, if you change dollars to tolars and then back again, so try to avoid changing more than you'll need. I wouldn't count on ATMs, though they are improving. On my first visit, I took traveler's checks.