Camping in Slovenia isn’t as easy as we had hoped because there are so few campgrounds. Camping is only legal in the official campgrounds, which are all commercial. There are about 45 of them, and they are more concentrated in the tourist areas. So although in some areas they are easy to find, sometimes they are few and far between. Apart from the rain, this was another factor which influenced the distances we rode each day. Sometimes we opted to stay in more expensive accommodation just to keep dry.
On the plus side, every campground we stayed in was of a pretty good standard. For the two of us we were usually paying around €15, and the pitches were all grassed.
At Maribor tourist info we picked up a brochure Camping in Slovenia, which listed all the campsites and which we referred to quite often.
In general I think it would be OK to free camp, although I don’t know if there is any punishment if you are caught. We did it once in a populated area and weren’t bothered by anyone.