Car boot sales/Flea markets???

I'm heading over to Krakow in October (from England) and I hope to buy a few simple polish language books/comics to bring back....as a kind of aid to help me in my ambition to learn Polish.
I'v checked the web but 'Little Monkey' and the few other Polish books for kids are quite expensive and its not clear whether some are Polish stories (in English) or in the Polish language (Dragon of Krakow/Crystal Mountan).

Where will I find books like these at reasonable prices?

If a Pole was asking me where in England they'd find kids books in English I would suggest Car boot sales/Flea markets and charity shops but I'm not sure whether these options are the best over in Poland.
SUGGESTIONS PLEASE.

allegro?

You can go to secon-hand book shop ( antykwariat ) but I wouldn't recommend antykwariats in the centre of Kraków. Kraków specializes in old or rare books, and the prices are rather high. It's just a bookworm's paradise, a perfect place to shop for books, but rather for high literature and rare scholarly stuff.
But if you happen to find just a normal antykwariat, then, you'll get cheap children books there. There must be some in Kraków but I don't know. When I'm there, I look for that more expensive stuff.

And the problem with Polish books for kids is that many of them are just crap.