During the German invasion of Poland, when Soviet Union invaded from the East and situation turned hopeless, Polish government left the country. After establishing government-in-exile in England, they commissioned design and print on new Polish banknotes to replace currency compromised under German occupation. 
This set of banknotes was printed in London in huge volumes, but was never used. Polish post-war puppet regime controlled by Moscow was mistrustful and not interested. As a result all were destroyed and 1000 of each given to the National Bank of Poland for musealpurpose. Only a 100 of each denomination were released for collectors in the 1970s making them extremely rare and valuable collectibles. 
Here you can purchase them in perfect unissued state freshly printed on high quality cotton paper (for realistic look and feel) by Krause Papierwerke.