How is difficult to find part time job for international students in Poland?
Practically impossible. Make your plans on the very safe assumption that you will never be able to earn a single penny during your stay in Poland. If you need to earn to learn, Poland is certainly not a viable option for you. You will run out of money and have to return to your home country before you finish your studies. Anyone who says that non-EU students can easily find work in Poland and support themselves during their studies is lying.