Auchan hypermarket in Krakow should have them for 5 or 10 ZL. If not near one, try any of the newsagents or little booths in shopping malls like Teletorium. But the newsagents should have one. Some other supermarkets also have them. Zabka (the frog logo convenience store) sometimes also have them but be sure to make clear you want the card not a top up. Say phonetically "Prosh-ah Virgin/vergeen mobeel seem carter?" and that should be enough to get you understood. Of course, the shop assistant might speak English so you might get the response "You want what, darlin'?"
