For example, when you were at school, how many friends among you were learning to play instruments..
Some (4 maybe 5, per 25 - 30 students) at class. From my school experience the most populat was guitar, some students were learning to play a piano, some - violin. At common polish schools not many polish children learn playing professionally, although sometimes they attend at private lessons.
At music primary or high schools children learn to play 2 chosen instuments by them and then they practise at home a lot, so I think that these are places where you will find students rather easily.
Also private schools, where parents pay for their kids learning, offer many additional courses including learning to play classical instuments.
A good option could be visiting some schools and giving out your cards to pupils, or talking to parents at "wywiadówka" (meetings teachers with their students parents which has a place once a month or twice a semester (it depends). Some teachers for sure will let you present your offer (don't forget to ask a headmaster for permission before).
Some parents will be delighted having a teacher who will not only teach their kids to play an instrument, but will be also a native speaker of a differrent language.