Until recently the best place to look for bus timetable information for Slovenia was the website of Ljubljana Bus Station. However this naturally has the drawback of only including routes that pass through Ljubljana. Information about other routes was scattered in the websites of Slovenia's numerous transport companies.
I've recently learned about www.vozni-red.si, an initiative of the Mountain Wilderness organisation to promote public transport. Using data from the Ministry of Transport, the site for the first time brings together the schedules of all domestic bus routes in one place.
You can input any two towns to find bus connections between them. Pay attention to the footnotes beneath the schedules as they often contain important information (vozi = runs, ne vozi = does not run, vsak dan = every day, vozi ob delavnikih razen sobote = runs on working days except Saturdays; for more vocabulary including the days of the week see the site's help page).
A few examples of schedules that previously were quite difficult to find:
Kranjska Gora to Bovec (over the Vršič Pass)
Bovec to Kobarid (in the Soča Valley)
Koper to Piran (along the Adriatic coast)