There is no need to buy a ticket in advance if you're taking regular buses as they go every day so you can buy your ticket as soon as you arrive to Zagreb for the following day (or any day you want to go on). I'd recommend taking a bus at 07:30 (arrival to Plitvice at 09:45) and getting a bus back at 17:50. If you're buying a return ticket you have to be aware that you can only use it on the same bus company (the above written times are for the same operator). If you're buying a one way ticket you can come back at the time that might suit you better. Check the timetables on www.akz.hr/En.htm (Plitvice in search is Plitvicka Jezera). If you're talking about a tour with an agency there are several and the largest one is on www.adriatica.net.
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