Upvote:3
There are many private companies running the buses. The Split buss station is near the south and of the port. There are several counters there and you can ask which ones go to Plitvice. Many but not all do. Hint: Many Plitvice buses have a photo of waterfalls printed on their side.
There should be no trouble at all buying bus tickets in Split but make sure to buy you return ticket too. Plitvice has no real bus stations and a bus stop is loosely defined as a segment of the highway which goes through the area. On one of the stops near our hotel in Plitvice, there was someone waiting for passengers but she was not of much help and did not want to use her phone to call for us when there were no buses or taxis arriving.
Upvote:6
There are many private bus companies on both the routes from Split to Plitivice and from Plitvice to Zagreb. Buses in Croatia normally don't sell out, though they may in particularly busy travel periods, I think you should be fine in October.
BalkanViaTor has done a pretty good job at collecting the schedules in one place. You can book tickets in advance for a few bus companies on GetByBus.
Here's some info on going from Split Airport to the station. Buying at the station in Split should be fine, in Plitivice you have to wait at unsuspecting bus stops near the park entrances and pay the attendant on the bus or driver - you should be able to find a bus no problem but you may need a little patience. You may be best off booking ahead with Autotrans bus company since they have several departures and market heavily to tourists, I'd think their buses might be easier to find in Plitivice, and if you miss one there's likely to be a later departure.