Try the Czech and Hungarian national rail company websites. The links are in a “sticky” at the top of this forum and under “transportation” on the “travel tips” tab. However you may not be able to buy this international ticket from either of those websites.

Why do you want to buy the ticket on-line? It will be reactively easy to buy it at any major train station in Europe. It’s very unlikely doubles will be sold out and you can almost always get some kind of accommodations up to about 2 hours before train time. You can also buy on-line from almost all of the travel agencies that sell rail passes — raileurope.com, rail pass.com, eurail.com and maybe here under the “bookings” tab. However, if you do that you’ll end up paying “global fare” which is about 30% more than you’d pay at the station, plus you may have shipping and handling fees. You will also probably miss any special fares that may be available from the national rail company websites.

Most European train tickets (Eurostar is an exception) can only be purchased 60, or in some cases 90, days in advance from any source.

Thanks for your advices.
It’s my first time going on a night train, was a bit worried for not getting a sleeping compartment…lol