Unfortunately, in Ukraine, I could not find a place where you could buy Zloty.So you have to buy euros or dollars, and with them go to Poland and there change them to PLN. It's not very convenient. Because already on arrival at the station Zloty is needed to get to place. For the first time we have bought 50 zlotys from conductor in train. In the morning we went to change the money in the exchanger.

Almost all exchangers have around 20 different currencies. The exchanger can be  recognized by the sign Kantor. You can also exchange the money in some banks. 

The approximate exchange rate can be checked on official web site of Polish National Bank. 

You are also free to use debit cards of your country.