I wish to travel by bus (probably) from Lake Bled to Zagreb on 20 June. When I get into the Bus Croatia site it tells me that tickets are not available on-line. Is it possible to purchase tickets elsewhere on-line or should I just leave it until I am over there?
You can buy them there. There are many busses per day to Ljibliana. Just go to the tiny little station. If you are concerned you could buy your ticket when you first get to Bled. No need to buy online. You might have to purchase the ticket from Ljubljana to Zagreb at the Ljubljana station, which is also easy. When we went a few years ago-the only direct bus from Ljubljana to Zagreb left Ljubljana at 5am.
There are also daily direct trains from Lesce-Bled to Zagreb, which take about as long as the bus, but you would also eliminate the wait in Ljubljana.
I don't like to buy international bus tickets on the day of travel since those buses do sell out occasionally, though there have been a lot of occasions when I have done so, and I've always gotten on the bus I wanted. I'd try to buy the onward ticket as soon as I arrived in Slovenia, and I'd choose whichever mode of transportation made that easier.