If you are in Kraków you will find there is a specialist wine shop on Dietla just next to the crossroads at Starowislna. Also a couple in Kazimierz. These are very expensive though so be prepared to pay through the nose.
I always found Lidl had a good selection as did my local Biedronka (I had 2 next to me, one had a great selection, the other not so much). In the latter I have found NZ Sauvingon for 17.99 at times. It is hit or miss as it depends the stock that's in at the moment, their prosecco is pretty good too. Also check out the Portugese Vinho Verde as that's dry with a nice effervescence and is often in stock. Lidl is a safer bet as their wine selection is more consistent, however I always find they are better for red than white, so keep that in mind.
Other than that, I echo the comments of others, make sure you are looking for "Wytrawne" and check the description on the bottle as you will often find Polwytrawne wines sitting in the Wytrawne section.