For international bus trips, you can buy tickets at the ticket office or travel agency. Eurolines is one of the largest providers of international lines, working together with its in-house subsidiary Slovak Lines.
For international lines operated by Eurolines, you can buy/book a ticket online on their website.
You can also upgrade to a Flixbus connection in Slovakia, a transport company that works with over 2500 travel destinations in 34 countries across Europe.
The student agency also provides several international and domestic routes with special prices for students (tickets can be bought online).