What date and time do you want to travel? 

Where did you try to make the reservation?

I´ve tried everywere… DB, ÖBB, DSB, Be, SNCF - also physically sales.

With no luck… Not even in München HBF is able to issue these…. :-( 

And they are mandatory...

You tried wrong palces and did not read far enough. In the general overview (click on help above) it is utmost clear; ONLY at counter in HRvatska

There are just a few trains, there are many more much quicker buses (pay), but the route seems very populair at the mo, so good luck with it

@Ulrich Albrechtsen  because Croatia didnt load their trains this year in the International booking systems. Because of that its only possible when in Croatia.