Posmysz herself suggested what those leaflets may have been in a lengthy 2017 interview with Michał Wójcik. While living in Krakow at age 18, Posmysz attended classes that were part of the underground school system that developed in Poland after the Nazis banned higher education for non-Germans. She recounts being arrested when the Gestapo entered her class one day and found objectionable literature (translations via Google):

Classes have started and at some point the door opens. They entered as normal, in civilian clothes. They weren't even aggressive. What are you doing? We couldn't answer. Someone said we were meeting. Where do you live? Everyone gave his address. What are you doing here? We talk about work because everyone works somewhere. I had a certificate that I worked in a casino. Did not help. One boy found leaflets. I think it was the "Orzel Bialy" [White Eagle] magazine. You're coming with us, they ordered.

"Orzel Bialy" refers to Zwiazek Orla Bialego (Association of the White Eagle), an underground paramilitary organization founded in Krakow in 1939 out of the remnants of an earlier group called the Rifleman's Association (Związek Strzelecki). Members participated in intelligence gathering and propaganda campaigns against the Nazis. The organization's stated purpose was to "conduct an active fight against the enemy flooding the country."

In the interview transcript, Wójcik inserts this note about Zwiazek Orla Bialego:

The organization issued "Nakazy Dnia" [Orders of the Day]. At the end of 1941, the Zwiazek Orla Bialego was broken, but it still managed to publish a few issues of the magazines "Nasz Sprawy" [Our Cases] and "Polska Walczy" [Poland Fights].

Those are three candidates for the publication the Gestapo found in Posmysz's classroom. All three are listed in the Catalog of the Krakow Underground Press 1939-1945 although only "Nakazy Dnia" lists Zwiazek Orla Bialego as its publisher. With regard to whether any copies still exist, the catalog (published in 1978) does provide an index of "preserved copies" of Nakazy Dnia on page 95. Someone who reads Polish would have to help you with deciphering the filing information and possibly locating these copies if you're interested. I can't find any digital copies. The only other detail I can find is that it was a weekly publication "intended for the intelligentsia" according to the files of the Government Delegation for Poland, part of Poland's government in exile during WW2.

It's important to note that although the Gestapo ultimately sentenced Zofia Posmysz to Auschwitz for distributing these materials, she claims she had no active involvement in producing or distributing any of these publications. In her interview, she explains that she first learned about the Polish underground organizations from some boys in her class who would bring around newspapers and magazines "probably to show off," and that she only really learned about them when she was arrested. Describing her arrest, she states:

The boys probably had not even the entire newspaper, but some exception, an extract. When the Germans found it, I understood that it could end badly. I was shocked. But then I did not think it would be so bad. They took us to Pomorska Street, to the Gestapo. First hearing. Where have you been? What you were doing? Where are these leaflets from? Where do you store them? I didn't know how to answer, I didn't know how to behave. I said that I don't know about any leaflets. Then the interviewer slapped me in the face. Shock, horror. They absolutely wanted me to admit these leaflets. In the end, I admitted to participate in the classes, but I couldn't get to these magazines. After all, I really had nothing to do with it.