In Bulgaria, at an international ticket office:

https://www.bdz.bg/en/a/stations#tid-11206

Maybe DB can also book a seat reservation.

DB = Deutsche Bahn, German Railways have also offices in Italy.

In Milan and Verona. 

If you are now in Italy you could call DB or write an e-mail to ask if they can sell it. 

Domande sui prezzi dei biglietti, sugli orari o sulle prestazioni di servizi

Qui è possibile ricevere informazioni sugli orari, sui prezzi dei biglietti e sui servizi offerti dalla Deutsche Bahn. 

Informazioni in italiano:
+39 02 67479578
(i costi dipendono dall'operatore)
da lunedí a venerdí dalle ore 09:00 alle 18:30, sabato dalle ore 09:00 alle 15:00

2018 I wrote an email to info@dbitalia.it, I don’t know if this adress is still in use. 

Depending on WHERE you are now in IT: DB/OeBB maintains its own offices on some major stazione where their MÜnchen/Austria trains stop (like Verona/Bolzano) and these have the normal DB-computers so could perhaps also help there.

But in general-except or some very busy days last summer-these trains are not very full and you do not need to worry weeks/monthes ahead about space.

DB told me they CANNOT book this train although they used to be able to. There are NO SEATS on this train only couchettes and sleepers cross the border. You must reserve several days in advance as this train can be fully booked. If travelling on a pass I recommend travelling back to Sofia and joining it there. This is because this train is sometimes heavily delayed getting to Plovdiv because of major engineering work between there and Sofia. You will be nice and snug on the train rather than freezing to death at Plovdiv. A sleeping berth is very good value. My wife and I paid the equivalent of 30 euros for a double sleeper when using our interrails. Incidentally we arrived three hours late in Halkali! This was in October this year.

Did they try to book using the train number search or only timetable-based? The latter method will fail for this train but unfortunately, too many DB staff don't know the first method and will claim it's not possible.

DB can book the seasonal couchette Bucharest - Istanbul using train number search. But it's also quite possible that Bulgarian railways (who will manage the reservations in this direction) don't share their reservation data anymore or something similar.

Hi - Obviously I can’t know for sure but the DB man I spoke to (on the phone in their Berlin office) seemed very knowledgeable and had reserved the train in the past. He booked our Wien - Videle (Romania) sleeper with no difficulty and we picked it up from a ticket machine in Munchen. It would be great to hear if anyone in the Community managed to get DB to book this train since it started running again this year. It may be that this a post pandemic change. Incidentally Romanian Railways can’t book this train either - although they could book the Bucharest couchette car when it was running. As with just about everything in Eastern Europe things are never that clear but a couple of retirees from GB loved our travels in Romania Bulgaria and Turkey this year!