There’s a nice selection of (mostly) postwar Polish posters at a Grayspace Poster Gallery.
I’ve set the background to white in the selections above, using the “remove background image” and “page color to white” bookmarklets (I realized afterwards that I should also have set the text to black, and I found the zap colors bookmarklet that both sets the background to white and the text to black). I like clarity. Not everyone does. Without the bookmarklets applied the site looks like this: