First thought: go to any Empik store, they usually have smaller or bigger English sections. Here's list of there stores: empik.com/pomoc/salony [wybierz miasto = choose the city] or list of books you can buy online: empik.com/obcojezyczne
I used to pick all kinds of English books at the second hand clothes stores, they source most of their clothes stock from the UK anyway so i guess they were picking up books from the same places and a few used to have hundreds of random English books. Mostly celebrity autobiographies and such but occasionally i got lucky with the kind of things that interest me.
Yes, about 1 in 5 secondhand clothes shops will usually have some English language books gathering dust somewhere near the CDs or bric-a-brac if they have a bric-a-brac section. If the OP's not fussy, he or she will find at least 3 or 4 books (typically novels but also stuff like Alan Carr's Stop Smoking and other random stuff) somewhere in each of those 1 in 5 shops.
Archives - 2010-2019 / Life / Are there any bookstores in Wroclaw, Rzeszów or Krakow where I can buy books in English?