Kassy10: Late answer, but if it can someone else help...
You can't buy reservation for this train online, but you can see online how much is this train booked out for specific day. Here is the guide.
At Czech Railways eshop site - www.cd.cz/azak/setlang?lang=en&sourceapp=https%3a%2f%2fwww.cd.cz%2feshop%2fdefault.aspx
- in section Czech Republic by Train you have to select Ticket, Reservation and submit by Buy Ticket button. Then you fill in From: Prague
, To: Buhumín
(last train station in Czech republic on your way), Time: 21:00
and select the date of your interest. After pressing Select on the train R 433 Šírava, you will see the page for buying domestic ticket. You shuld select the tab Only reservation and then the link Select your seat. Now, you will see the booking status of whole train, every carriage including couchettes. Booked seats/couchettes are maked red.
I hope that can help you decide which time before the way you shuld book you seats. ;-)