What is your proposed itinerary? Unless you have a complicated or unusual itinerary, you may not need a special map. On my very first trip to Croatia, I bought a huge fold-up map for driving between Zagreb, Plitvice, Split, Mostar, and Dubrovnik, but I never even needed it. I learned the basic route ahead of time, the roads are well signed (much of that route is a modern expressway), and driving there was easy. Had I ventured off the main roads I might have had more difficulty.
Driving in Slovenia a few years later, I didn't bother with the big map - I got away with a free map from the tourist office. I drove from Ljubljana to Skofja Loka to Bled, over the Vrsic Pass, down to Kobarid, to Lipica, and eventually to Piran. I also did a little research ahead of time, printed out some Google Maps, but again, driving was easy. I never really got lost. Roads are well signed; there are signs pointing to the next town (off the expressways) and once you start thinking that way, navigating is pretty easy.
In 2015 I had my Garmin GPS when I drove across Croatia, Montenegro, and Bosnia. I went much more off-road this time, and my GPS wasn't always right (especially in Bosnia) but I managed just fine. Earlier this year, I used my smart phone with Google Maps as a GPS for driving for just a day in Slovenia - worked beautifully.