We’ve been to both Poland and Czechia a couple of times (last there 2018 though). Never had a problem finding campsites, except once on what turned out to be a holiday weekend in Poland. No pitches left but did have a lodge thing, so had that. Very nice.
In Czechia we stayed at the Skoda owners club campsite in Kosmonosy which was interesting. Cheap & cheap beer. Just to be clear, I have no interest in Skodas, just that we turned up in the town and followed signs to a campsite.
www.autokemp-kosmonosy.cz
Also here, in Pilsen where we’ve stayed twice so it must have been at least ok. Also cheap beer and a great sausage based menu in the cafe.
bolevak.eu
There were others but I can’t remember where but I don’t remember anywhere being a dud.
Poland, not quite so well blessed with campsites in our experience. But that could have just been bad luck. We camped on the infield of a sports arena somewhere or other, which was surprisingly good, and a really nice one hard on the border with Slovakia in the Tatar mountains. Although massive thunderstorms made for an interesting night and a river getting increasingly swollen and threatening to overflow. Cheap laundry though, but an expensive restaurant on site where we ate due to the storms. The lodge mentioned above, and somewhere okish on the coast. On reflection we didn’t do Poland justice so definitely need to go back.