These people might be useful for you: postermuseum.pl
A source for genuine ones is Allegro, rather like eBay but specific to Poland.
This one is a museum of caricatures rather than posters, however I did go to a poster exhibition there a while ago: muzeumkarykatury.pl/joomla/
These people: polskiplakat.link2.pl sell posters, and are also connected with the old film posters on display in the pedestrian crossing under pl. na Rozdrożu mentioned here: culture.pl/pl/miejsce/galeria-plakatu-plac-na-rozdrozu-w-warszawie