I am told that French suited cards are most widely used. I believe that the 32 card German suited pack (William Tell pattern) is also used to some extent, for example for Preference, which is played under similar rules to the Croatian version of the game.
A standard French-suited pack, sometimes reduced to 40 cards by removing the pictures, is used for the fishing game Tablić.
Slovenia also has its own version of Tarok, played with a 54 card French suited tarok pack similar to that used in Austria (22 trumps, four suits of 8 cards).
In the region known as Primorska, near the Italian border, a 40-card Italian suited pack of the Trieste pattern is used to play Briškula (equivalent to the Italian game Briscola) and Trešet (equivalent to Tressette).
The World Casino Directory includes a listing of Casinos in Slovenia.