What's your travel date and departure time?

Booking in Budapest will surely be possible (if there are still seats), and also in many other ticket offices, doesn't need to be in Hungary.

You can book it at ticket counters or also on cd.cz for 74 CZK (3€).

Look for your connection and select purchase reservations only.

That reservation is optional in 2nd class by the way.

The Interrail app says "seat reservations required".
Does that mean I can just get on and don't have to reserve a seat? 
I don't care if I have to sit on the floor. the main thing is cheap

You’ll find that the app is quite often wrong. Trust on the advice of seasoned train travelers on the forum ;)

Just find a free seat for this train, and you’re good.