I have only read half of Snyder's books, but I think that OP's skepticism is misguided.
The OP is in error interpreting "posters" as propaganda items for mass-distribution, and the efforts for identifying them are on a wrong path. Snyder is not suggesting that there were cases where, for propaganda purposes, Stalin's figure was superposed with a swastika etc. The "posters" in question are in fact banners like those at the joint military parade of Brest-Litovsk where the leaders' effigies and/or specific symbols of the two powers were jointly displayed.
The fragment quoted is from an article to which I do not have full access, but it is similar to a passage in a book by Snyder that I do have, namely Black Earth - The Holocaust as History and Warning, chapter 7, Germans, Poles, Soviets, Jews (I have it as an e-book with no standard pagination):
In 1939 and 1940, the Soviet alliance with Nazi Germany sowed
ideological confusion among Soviet citizens. The Soviet press ceased
to criticize German policies and began to publish Nazi speeches.
Soviet citizens in public meetings occasionally misspoke, praising
“Comrade Hitler” when they meant “Comrade Stalin” or calling for “the
triumph of international fascism.” Swastikas began to appear as
graffiti in Soviet cities.
The fact that there is no direct scholarly reference for all those details is not bothering me too much, I can trust the scholar on this, namely that on the occasion of the Nazi-Soviet alliance:
- parts of Nazi speeches were published/quoted in the Soviet press
- Swastikas appeared as grafitti in the USSR
- Swastikas and communist symbols appeared together along the posters of the two leaders on certain occasions
In chapter 4, The State destroyers, we find this:
German and Soviet forces met at Brest and organized a joint victory
parade, swastika followed by hammer and sickle, “Deutschland über
Alles” followed by the Internationale. The Soviet commander invited
German reporters to visit him in Moscow after the common “victory over capitalist Albion.”
The above is what is meant by the passage in the article that intrigued the OP:
Swastikas began to appear on buildings or even on posters of Soviet leaders:
- On the occasion of the joint victory parade in Brest, there were joint posters of the Nazi and Soviet leaders and symbols (swastika and hammer&sickle); that doesn't refer to popular propaganda posters (meant for large distribution) like the ones the OP was looking for, but only to local and specific parade posters, probably meant to appear only once, then and there. But that doesn't mean that propaganda items of larger distribution celebrating the Nazi-Soviet cooperation were totally absent: see below on swastikas-material distributed (exclusively or not) to ethnic Germans.
That type of "posters" celebrating the rapprochement of the two powers
must be imagined as obligatory in all joint festivity of that period. In the best known book by Snyder, Bloodlands. Europe between Hitler and Stalin, Chapter 3, National terror, p.116, we find this:
Ribbentrop made for Moscow, where, as both Orwell and Koestler noted,
swastikas adorned the airport of the capital of the homeland of
socialism. This, the final ideological shock that separated Koestler
from communism, was really a sign that the Soviet Union was no longer
an ideological state.
The larger passage makes reference in NOTE 58 to
Haslam, Collective Security, 227 [meaning: Jonathan Haslam, The Soviet Union and The Struggle for Collective Security in Europe, 1933-39], Quotation:
Weinberg, World at Arms, 25, [A World at Arms: A Global History of World War II] but mainly this refers to eyewitness reports made by both George Orwell and Arthur Kostler.
In NOTE 43, p.487 of the same chapter we find this:
The Soviet rationale was a classic one. First, the NKVD “established”
that Germany had hundreds of spies among the Volga Germans. Then, the
NKVD argued that the entire population was guilty, since none of the
Volga Germans had reported all of this espionage to the proper
authorities. In a particularly refined move, the NKVD used the
presence of swastikas in German households as evidence of Nazi
collaboration. In fact, the Soviets had themselves distributed those
swastikas, in 1939, when Moscow and Berlin were allies, and a friendly
visit from Hitler was expected.
That means that swastikas were used at some point during the 1939-41 pact in Soviet propaganda aimed at the Soviet ethnic Germans. When these people became victims of later persecution, those pro-Nazi propaganda items were unsurprisingly used against them.
I see nothing surprising here, although surprise is the main base of OP's skepticism.
That surprise is an impression produced by an article that is judged out of the context of Snyder's main argument in more than one book: that the Soviet-Nazi common (simultaneous) actions, but also subsequent (non-simultaneous) actions on the same territories (partition of Poland and other parts of Eastern Europe, but also the occupation first by the Nazi and then by the Soviet - or the other way around - of much of Eastern Europe) have to be considered the main explanatory factors for the specific (chronological and geographical) development of the most tragic events of the WW2 (the quasi-total destruction of the Jews, and the death of millions of Poles, Ukrainians, Belorussians, and Russians), as well as for the unprecedented realities of post-war Eastern Europe, which some now take for granted (the sudden general absence of the Jewish communities which had flourished there for hundreds of years, and the delimitation of new frontiers in relation to the ethnic cleansing of Poles of Ukraine, of Ukrainians of Poland, of Germans of Poland and Czekoslovakia, etc).