You can buy this ticket on the day of departure. If you would like to buy a seat reservation, than you can buy it at least 10 minutes prior to scheduled departure of the train form your station.This
sentence (For IC, EC and fast trains departing from Slovakia sales close 2 hours prior to scheduled departure from their originating station) is validated just for a few international trains, but no worries, it not validated for trains from Bratislava to Budapest.
Price of the one-way ticket for the route Brattislava-Budapesť is 16,50 EUR for an adult in the 2nd class. Special price of the return ticket for the route Bratislava-Budapesť is 17,50 EUR for an adult in the 2nd. class. You can buy tickets just at ZSSK cash desks ( train station with system KVC ). It is not possible to do purchase online.
Купить все можно в кассе в день отправления. Никаких проблем. Но вот стоимость 16,5 евро в одну сторону, а если брать туда-обратно, то 17,5! Итого проще взять на сайте Венгерских ЖД туда обратно и распечатать, чтобы в кассе на вокзале в Братиславе не стоять