Since most (all?) computer museums can qualify as retrocomputing museums, you might find this list of computer museums useful: https://en.wikipedia.org/wiki/List_of_computer_museums
You could also look at science museums and see whether they have some computer section. E.g. the large Polytechnic Museum in Moscow, Russia has a nice computer section:
There is a computer museum in Halle, Germany, formerly in the GDR. They have a lot of old computers and other devices on display from that time and place : http://rechenwerk-halle.de/usr/digital-ag/projekte/andere/museum/ or http://rechenwerk-halle.de/usr/digital-ag/RECHENWERK/index.html
If you google "Robotron museum" or "computer museum DDR&...
Yandex Museum, in the headquarters of Yandex -- Moscow, Timura Frunze st., 11 str. 13.
Not a very large one, but has a dozen or so of old computers, from around the world, not only from Soviet block.
Website (in Russian): https://yandex.ru/museum/
They also have a branch in St. Petersburg: https://yandex.ru/museum/spb (apparently currently closed, but to ...
There is a computer museum in Sigulda, Latvia.
It's a private project by a single enthusiast (known as MaiklsBlack on the net). He's been collecting devices for years and the museum has reportedly grown to 90 m^2 in size. He reports to have had over 380 old computers in 2018, about 80% of them working. His sites are only available in Latvian and Russian ...
There is a small computer museum in Brno, Czech Republic: https://www.fit.vut.cz/units/museum/.en
It focuses primarily on machines manufactured in Czechoslovakia (and some imported ones).
Also in Brno, there is an exposition in Technical museum about old computers: https://www.tmbrno.cz/en/expozice/computer-technology/
Some of the most interesting pieces include the first robotic animal ever constructed https://en.wikipedia.org/wiki/Ladybird_of_Szeged and the originator of the once ubiquitous 3.5" floppy drive https://ajovomultja.hu/mcd-1-casette-floppy-marcell-janosi?language=en (the English translation is ...
Regarding Bulgaria you could visit the "National Polytechnic Museum" http://polytechnic-museum.com/. There are a mix of different technologies, a few years ago they had an explicit 'old pcs' exhibition.
I can recommend the computer games museum in Berlin (https://www.computerspielemuseum.de/). It is in the former GDR part of Berlin, and because you are interested in retrocomputing I'm sure you will enjoy it.
The Computerspielemuseum in East Berlin has already been mentioned, but there are also some old computers or reproductions (including a reproduction of one of Konrad Zuse's first computers) in the Technikmuseum in Berlin. The Technikmuseum is in the Western part of Berlin, but if you happen to be in town anyway, you should not miss it.
Dresden was the center ...
Half the answers seem to be from Bulgaria ¯_(ツ)_/¯
I think there was some kind of specialization between the countries and we had computing production. It didn't work well in the long term though.
Here is a video from museum in Kilifarevo for old tech, not only computers:
Update: I see that retro-...
There is The Museum of Computing and Informatics in Riga, Latvia.
To inform the society about the history and development of the computing, the Institute of Mathematics and Computer Science, University of Latvia, since 1984 houses The Museum of Computing and Informatics. It's the only museum devoted to computer science in Baltic. Museum's permanent ...