There are car hire places at all the airports listed on the flights tab above. If you hire in Austria or Italy, you need to ask if there's any extra charge to take the car over the border.
At Ljubljana Jože Pučnik Airport,
Argus Car Hire seem to offer the best deals, but there are a few others to try on the
Airport Car Hire page. As always, check that the rate includes unlimited kilometres, insurance (CDW & TP), tax, airport surcharge; and check the excess in case of an accident.
Be aware that by law you need to keep your main headlights on all day when driving anywhere in Slovenia. You need to have a 'vignette' sticker in your window to use the expressways, but this should be provided with the car.
Roads in Slovenia are generally excellent. The roads up to Bohinj and Kranjska Gora are fast enough, and there are expressways to Ljubljana, Maribor, Villach (Austria), Trieste (Italy), and some of the way to Zagreb (Croatia).